gnu: Move most packages from guile.scm to new module.

* gnu/packages/guile.scm (artanis, guildhall, guile-aspell, guile-bash,
guile-8sync, guile-daemon, guile-dsv, guile-fibers, guile-syntax-highlight,
guile-sjson, guile-colorized, guile-pfds, guile-aa-tree, guile-simple-zmq,
jupyter-guile-kernel, guile-sparql, guile-debbugs, guile-email,
guile-debbugs-next, guile-newt, guile-mastodon, guile-parted, guile-xosd,
guile-dbi, guile-dbd-sqlite3, guile-config, guile-hall, guile-ics, guile-wisp,
guile-sly, g-wrap, guile-miniadapton, guile-reader, guile2.2-reader,
guile-ncurses, guile-ncurses/gpm, guile-lib, guile2.0-lib, guile2.2-lib,
guile-minikanren, guile2.0-minikanren, guile2.2-minikanren, guile-irregex,
guile2.0-irregex, guile2.2-irregex, haunt, guile2.0-haunt, guile2.2-haunt,
guile-redis, guile2.0-redis, guile2.2-redis, guile-commonmark,
guile2.0-commonmark, guile2.2-commonmark, mcron, mcron2): Move these variables
from here...
* gnu/packages/guile-xyz.scm: ...to this new file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/installer.scm,
gnu/packages/bioinformatics.scm,
gnu/packages/ci.scm,
gnu/packages/gtk.scm,
gnu/packages/guile.scm,
gnu/packages/mail.scm,
gnu/packages/package-management.scm,
gnu/packages/skribilo.scm,
gnu/packages/web.scm,
gnu/services/mcron.scm: Update module references.

gnu: %default-xorg-modules: Include xf86-video-amdgpu.

* gnu/services/xorg.scm (%default-xorg-modules): Include xf86-video-amdgpu.
Rephrase comment to clarify the priority order.

gnu: %desktop-services: Use polkit-service-type.

* gnu/services/desktop.scm (%desktop-services): Replace polkit-service
with polkit-service-type.

gnu: %base-services: Use *getty-service-type.

* gnu/services/base.scm (%base-services): Replace agetty-service with
agetty-service-type, mingetty-service with mingetty-service-type.

gnu: %base-services: Use login-service-type.

* gnu/services/base.scm (login-service-type)[default-value]: New field.
(%base-services): Replace login-service with login-service-type.

services: %base-services: Use udev-service-type.

* gnu/services/base.scm (udev-configuration): Fix typo.
(udev-service-type)[default-value]: New field.
(%base-services): Replace udev-service with udev-service-type.

services: %base-services: Use nscd-service-type.

* gnu/services/base.scm (nscd-service-type)[default-value]: New field.
(%base-services): Replace nscd-service with nscd-service-type.

services: Use guix-service-type.

* gnu/services/base.scm (%base-services): Replace guix-service with
guix-service-type.
* gnu/system/examples/docker-image.tmpl (services): Same.

services: openssh: Add escape hatch.

* gnu/services/ssh.scm (<openssh-configuration>)[extra-content]: New field.
* doc/guix.texi (Networking Services): Document it.

services: herd: Allow to pass arguments to start-service.

* gnu/services/herd.scm (start-service)[arguments]: New optional argument.

services: kmscon: Remove virtual-terminal requirement.

kmscon does not require that virtual terminals run in UTF-8 mode.

* gnu/services/base.scm (kmscon-service-type): Remove virtual-terminal from
requirement list.

services: avahi: Deprecate the 'avahi-service' procedure.

* gnu/services/avahi.scm (<avahi-configuration>): Export getters.  Add
default values.
(avahi-service-type)[default-value]: New field.
(avahi-service): Mark as deprecated.
* gnu/services/desktop.scm (%desktop-services): Use the 'service' form
instead of calling 'avahi-service'.
* gnu/tests/base.scm (%avahi-os): Likewise.
* doc/guix.texi (Base Services): Adjust example accordingly.
(Networking Services): Update accordingly.

services: Deprecate a few more service procedures.

These procedures were already either undocumented (and de facto
deprecated) or documented as deprecated or redundant.

* gnu/services/base.scm (guix-service, guix-publish-service): Mark as
deprecated.
* gnu/services/mcron.scm (mcron-service): Likewise.
* gnu/services/networking.scm (tor-service): Likewise.
* doc/guix.texi (Scheduled Job Execution): Remove 'mcron-service' and
adjust example.
(Networking Services): Remove 'tor-service'.
* gnu/tests/base.scm (%mcron-os): Use 'mcron-service-type' instead of
'mcron-service'.
* gnu/tests/networking.scm (%tor-os): Use 'tor-service-type' instead of
'tor-service'.
* tests/guix-system.sh: Likewise.

services: docker: Update comment.

* gnu/services/docker.scm (docker-shepherd-service): Update comment.

services: docker: Use more minimal service requrements.

* gnu/services/docker.scm (docker-service-type)[requirement]: Add
file-system-/sys/fs/cgroup/blkio, file-system-/sys/fs/cgroup/cpu,
file-system-/sys/fs/cgroup/cpuset, file-system-/sys/fs/cgroup/devices,
file-system-/sys/fs/cgroup/memory.  Remove elogind.

services: docker: Clarify service-extension shepherd-root-service-type.

* gnu/services/docker.scm (docker-service-type)[extensions]: Clarify
service-extension shepherd-root-service-type.

services: Use 'define-deprecated' for service procedures.

* gnu/services/base.scm (urandom-seed-service, gpm-service): Define
using 'define-deprecated'.
* gnu/services/messaging.scm (bitlbee-service): Likewise.
* gnu/services/networking.scm (dhcp-client-service): Likewise.
(ntp-service): Likewise.
* gnu/services/xorg.scm (slim-service): Likewise.
(gdm-service): Likewise.

services: gdm: Add default value.

* gnu/services/xorg.scm (<gdm-configuration>)[x-server]: Add default
value.
(gdm-service-type)[default-value, description]: New fields.
(gdm-service): Mark as deprecated.

services: docker: Depend on elogind.

* gnu/services/docker.scm (docker-shepherd-service)[requirement]: Add elogind.

services: docker: Specify log file for containerd.

* gnu/services/docker.scm (containerd-shepherd-service): Specify log file
for containerd.