gnu: glibc: Comment on the parallel build race.

* gnu/packages/base.scm (glibc): Augment comment regarding sequential
  builds.

gnu: Add tree.

* gnu/packages/admin.scm (tree): New variable.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu: valgrind: Update to 3.10.1.

* gnu/packages/valgrind.scm (valgrind): Update to 3.10.1.

gnu: gnupg: Remove #:prefix for (gnu packages compression).

* gnu/packages/gnupg.scm: Remove #:prefix for (gnu packages
  compression).  Add #:prefix for (gnu packages license).

lint: Add tests for the 'source' checker.

* guix/scripts/lint.scm (check-source): Export.
* tests/lint.scm (%null-sha256): New procedure.
  ("source: 200", "source: 404"): New tests.

tests: Fix import.

* tests/lint.scm: Use 'url-fetch' from (guix download), not (guix build
  download), although this was actually harmless here.

store: Attempt to decode build logs as UTF-8.

* guix/serialization.scm (read-maybe-utf8-string): New procedure.
* guix/store.scm (process-stderr): Use it for the build log and errors.
* tests/store.scm ("current-build-output-port, UTF-8",
  "current-build-output-port, UTF-8 + garbage"): New tests.

serialization: Factorize 'read-byte-string'.

* guix/serialization.scm (read-byte-string): New procedure.
  (read-string, read-latin1-string): Use it.

gnu: lftp: Don't save unknown SSH host fingerprints to known_hosts by default.

* gnu/packages/patches/lftp-dont-save-unknown-host-fingerprint.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/ftp.scm (lftp): Add patch.

gnu: Add zathura comicbook plugin.

* gnu/packages/pdf.scm (zathura-cb): New variable.

gnu: Add zathura postscript plugin.

* gnu/packages/pdf.scm (zathura-ps): New variable.

gnu: Add zathura DjVu plugin.

* gnu/packages/pdf.scm (zathura-djvu): New variable.

gnu: Add zathura PDF plugin.

* gnu/packages/pdf.scm (zathura-pdf-poppler): New variable.

gnu: Add zathura.

* gnu/packages/pdf.scm (zathura): New variable.
* gnu/packages/patches/zathura-plugindir-environment-variable.patch: New file.
* gnu-system.am: Add it.

gnu: Add djvulibre.

* gnu/packages/djvu.scm (djvulibre): New variable.
* gnu-system.am: Add the file.

gnu: Add muparser.

* gnu/packages/maths.scm (muparser): New variable.

gnu: ruby: Update to 2.2.1.

* gnu/packages/ruby.scm (ruby): Update to 2.2.1.

gnu: Add Russian Aspell dictionary.

* gnu/packages/aspell.scm (aspell-dict-ru): New variable.

gnu: Move 'which' to (gnu packages base).

* gnu/packages/which.scm: Remove.
* gnu-system.am (GNU_SYSTEM_MODULES): Remove it.
* gnu/packages/base.scm (which): New variable.
* gnu/packages/aspell.scm, gnu/packages/audio.scm,
  gnu/packages/autogen.scm, gnu/packages/cdrom.scm,
  gnu/packages/compression.scm, gnu/packages/datamash.scm,
  gnu/packages/games.scm, gnu/packages/glib.scm,
  gnu/packages/gnu-pw-mgr.scm, gnu/packages/gnutls.scm,
  gnu/packages/gps.scm, gnu/packages/guile.scm,
  gnu/packages/java.scm, gnu/packages/libevent.scm,
  gnu/packages/linux.scm, gnu/packages/lisp.scm,
  gnu/packages/node.scm, gnu/packages/ntp.scm,
  gnu/packages/orpheus.scm, gnu/packages/pciutils.scm,
  gnu/packages/python.scm, gnu/packages/qt.scm,
  gnu/packages/rsync.scm, gnu/packages/sawfish.scm,
  gnu/packages/scheme.scm, gnu/packages/ssh.scm,
  gnu/packages/statistics.scm, gnu/packages/texlive.scm,
  gnu/packages/wdiff.scm: Adjust accordingly.

gnu: Avoid #:prefix when importing a (gnu packages …) module.

* gnu/packages/admin.scm, gnu/packages/emacs.scm,
  gnu/packages/gnutls.scm, gnu/packages/gsasl.scm,
  gnu/packages/linux.scm: Remove uses of #:prefix for package modules.