gnu packages: Avoid description lines with leading "(".

* gnu/packages/curl.scm (curl): Reformat description to avoid a line
  beginning with "(" because such lines break font-lock highlighting
  in Emacs.
* gnu/packages/flashing-tools.scm (dfu-programmer): Likewise.
* gnu/packages/games.scm (gnugo): Likewise.
* gnu/packages/gnome.scm (json-glib): Likewise.
* gnu/packages/image.scm (jbig2dec): Likewise.
* gnu/packages/maths.scm (units): Likewise.
* gnu/packages/pdf.scm (mupdf): Likewise.
* gnu/packages/web.scm (perl-encode-locale): Likewise.
* gnu/packages/wordnet.scm (wordnet): Likewise.
* gnu/packages/xml.scm (libxml2):Likewise.

gnu: Document xorg packages.

* gnu/packages/xorg.scm: Add/improve 'synopsis'/'description' fields of
  many packages.

Revert "gnu: calibre: Update to 2.30.0."

The new version requires the SSL package from Python >= 2.7.9.

This reverts commit d5711017707b8e0e8a75c2e0b535fd73b8d78c6c.

publish: Write hashes in nix-base32 format.

* guix/scripts/publish.scm (narinfo-string): Use
  'bytevector->nix-base32-string', not 'bytevector->base32-string'.

syscalls: Struct deserializer can now return arbitrary objects.

* guix/build/syscalls.scm (read-types): Add RETURN and VALUES parameters.
  (define-c-struct): Add WRAP-FIELDS parameter and pass it to 'read-types'.
  (sockaddr-in, sockaddr-in6): Add first argument that uses
  'make-socket-address'.
  (read-socket-address): Remove 'match' on the result of 'read-sockaddr-in'
  and 'read-sockaddr-in6'.

download: Remove spurious warning about 'https_proxy'.

* guix/build/download.scm (open-connection-for-uri)[with-https-proxy]: Warn
  about 'https_proxy' only when 'getenv' returns a non-empty string.

file-systems: Subsystem cgroups now depend on /sys/fs/cgroup.

* gnu/system/file-systems.scm (%control-groups): Define 'parent' variable.
  Initialize the 'dependencies' field for all the subsystems.

file-systems: Add a 'dependencies' field to <file-system>.

* gnu/system/file-systems.scm (<file-system>)[dependencies]: New field.
* gnu/system.scm (other-file-system-services)[requirements]: Honor
  'file-system-dependencies'.
* doc/guix.texi (File Systems): Document it.

gnu: Add Linux-Libre 4.0.8.

* gnu/packages/linux.scm (linux-libre-4.0): New variable.

system: Use the CRDA udev rules.

* gnu/services/base.scm (%base-services): Add CRDA to the #:rules of
  'udev-service'.
* gnu/system/install.scm (installation-services): Likewise.

gnu: Add CRDA.

* gnu/packages/linux.scm (crda): New variable.

gnu: Add wireless-regdb.

* gnu/packages/linux.scm (wireless-regdb): New variable.

licenses: Add Fontana's copyleft-next.

* guix/licenses.scm (copyleft-next): New variable.

gnu: Add eog.

* gnu/packages/gnome.scm (eog): New variable.

gnu: Add exempi.

* gnu/packages/freedesktop.scm (exempi): New variable.

gnu: duplicity: Update to 0.6.26.

* gnu/packages/backup.scm (duplicity): Update to 0.6.26.

gnu: Add emacs-pdf-tools.

* gnu/packages/emacs.scm (emacs-pdf-tools): New variable.

Co-authored-by: Alex Kost <alezost@gmail.com>

system: Add 'kernel-arguments' field.

* gnu/system.scm (<operating-system>)[kernel-arguments]: New field.
  (operating-system-grub.cfg): Honor it.
  (operating-system-parameters-file): Add 'kernel-arguments' to the parameters
  file.
* guix/scripts/system.scm (previous-grub-entries)[system->grub-entry]: Read
  the 'kernel-arguments' field of the parameters file, when available.
* gnu/system/vm.scm (system-qemu-image/shared-store-script):
  Use (operating-system-kernel-arguments os) in '-append'.
* doc/guix.texi (operating-system Reference): Document it.

system: Fix typo in 'PS1' in skeleton '.bashrc'.

* gnu/system/shadow.scm (default-skeletons): Move misplaced backslash.

gnu: Add gnome-common.

* gnu/packages/gnome.scm (gnome-common): New variable.