gnu: Improve error reporting of the use-.*modules macros.

Suggested by Julien Lepiller and myglc2
at <https://lists.gnu.org/archive/html/guix-devel/2017-11/msg00106.html>.

* gnu.scm (%try-use-modules): New procedure.
(package-module-hint, service-module-hint): New procedures.
(try-use-modules): New macro.
(use-package-modules, use-service-modules, use-system-modules): Use it.
* tests/guix-system.sh: Test it.

services: Add 'lookup-service-types'.

* gnu/services.scm (lookup-service-types): New procedure.
* tests/services.scm ("lookup-service-types"): New test.

services: 'fold-service-types' includes (gnu services).

* gnu/services.scm (all-service-modules): New procedure.
(fold-service-types): Use it for the default MODULES value.

services: 'fold-service-types' honors its seed.

* gnu/services.scm (fold-service-types): Use SEED instead of '().

ui: Define and honor '&error-location' and '&fix-hint' conditions.

* guix/utils.scm (&error-location, &fix-hint): New condition types.
* guix/ui.scm (report-load-error): Handle them.
(call-with-error-handling): Honor '&error-location'.

ui: Introduce (guix i18n).

* guix/ui.scm (G_, N_, _P, %gettext-domain, %package-text-domain): Move
to...
* guix/i18n.scm: ... here.  New file.

gnu: service: Update comment.

* gnu/services/base.scm (guix-activation): Update comment.

install: Use overlayfs instead of unionfs.

I dediced to keep adding /tmp as tmpfs since I was not able to trigger bug
while still using unionfs, so I could not verify whether this mount is still
needed with overlayfs.  Mapping /tmp to tmpfs does not harm, so we are on the
save side.

* gnu/system/install.scm (make-cow-store): Mount /gnu/store without additional
  read-only bind-mount, since in overlayfs the "lower" level is always
  read-only. Add work-dir required by overlayfs. No need to sleep anymore
  since now using the mount syscall. [unionfs]: Remove now unused function.
  (%installation-services): Update comment.
  (installation-os)[file-systems]: Update comment.

build: Use overlayfs instead of unionfs.

Overlayfs is part of the kernel, while unionfs needs FUSE.  This also reduces
the size of the initrd by ca. 4.3% (487K).

* gnu/build/linux-boot.scm (mount-root-file-system): Remove optional parameter
  "unionfs"; mount using overlayfs instead of unionfs; new directory layout
  requied by overlayfs; update documentation.
  [mark-as-not-killable]: Remove now unused function

* gnu/system/linux-initrd.scm (file-system-packages): Remove now unused
  packages "unionfs-fuse/static" and thus unused related 'if'.
  (linux-modules): Replace "fuse" by "overlay".

gnu: miso: Use pypi-uri.

* gnu/packages/bioinformatics.scm (miso)[source]: Use pypi-uri.

gnu: Add r-vcd.

* gnu/packages/cran.scm (r-vcd): New variable.

gnu: Add r-laeken.

* gnu/packages/cran.scm (r-laeken): New variable.

doc: Move paragraph about signature verification to the top.

* doc/contributing.texi (Submitting Patches): Remind contributors to verify
cryptographic signatures at the very beginning.

gnu: fastcap: Use texlive-tiny.

* gnu/packages/engineering.scm (fastcap)[native-inputs]: Replace texlive with
texlive-tiny.
[arguments]: Fix indentation; set HOME in "make-pdf" phase to avoid font
errors with texlive-tiny.

gnu: librecad: Fix build.

* gnu/packages/engineering.scm (librecad)[arguments]: Add phase
"patch-boost-error".

gnu: Add r-performanceanalytics.

* gnu/packages/cran.scm (r-performanceanalytics): New variable.

gnu: Add r-xts.

* gnu/packages/cran.scm (r-xts): New variable.

gnu: Remove "open source" from descriptions.

All software in Guix is Free Software.

* gnu/packages/aidc.scm (libdmtx)[description]: Remove "open source" string.
* gnu/packages/firmware.scm (seabios)[description]: Likewise.
* gnu/packages/libupnp.scm (libupnp)[description]: Likewise.
* gnu/packages/maths.scm (cubicle)[description]: Likewise.
* gnu/packages/mingw.scm (mingw-w64)[description]: Likewise.
* gnu/packages/game-development.scm (openmw)[description]: Likewise.
[synopsis]: Remove "Free software".

gnu: pcb: Update to 4.0.2.

* gnu/packages/engineering.scm (pcb): Update to 4.0.2.
[arguments]: Add build phase "fix-check-for-display".

gnu: time: Update license.

* gnu/packages/time.scm (time)[license]: Change to gpl3+.