emacs: Add "Packages" option for 'guix challenge' popup.

* emacs/guix-command.el (guix-command-rest-argument): Add "challenge".

emacs: Add shell completions for 'guix challenge'.

* emacs/guix-pcomplete.el (guix-pcomplete-complete-command-arg): Add
  "challenge" to complete package names for it.

gnu: gcj: Run libjava tests.

* gnu/packages/gcc.scm (gcj)[native-inputs]: Add dejagnu.
  [arguments]: Enable tests and set test target to
  "check-target-libjava".

gnu: guile-next: Update to 20151025.e5bccb6

* gnu/packages/guile.scm (guile-next): Update to 20151025.e5bccb6.

gnu: subread: Use SSE optimizations on x86_64 only.

* gnu/packages/bioinformatics.scm (subread)[arguments]: Override CC and
  CCFLAGS conditionally dependent on target system.

gnu: crossmap: Update to 0.2.1.

* gnu/packages/bioinformatics.scm (crossmap): Update to 0.2.1.

gnu: Add yapet.

* gnu/packages/password-utils.scm (yapet): New variable.

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

gnu: Add xcompmgr.

* gnu/packages/xorg.scm (xcompmgr): New variable.

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

profiles: Add generation manipulation procedures.

* guix/scripts/package.scm (delete-generations): Use
  'delete-generation*' instead of 'delete-generation'.
  (guix-package)[process-actions]: Use 'roll-back*' instead of
  'roll-back' and 'switch-to-generation*' instead of
  'switch-to-generation'.
  (link-to-empty-profile, switch-to-generation,
  switch-to-previous-generation, roll-back, delete-generation): Move
  to...
* guix/profiles.scm: ... here.  Adjust to not print messages and to
  return values that can be used by user interfaces.
* guix/ui.scm (display-generation-change, roll-back*,
  switch-to-generation*, delete-generation*): New procedures.

utils: Add 'switch-symlinks', moved from (guix ui).

* guix/ui.scm (switch-symlinks): Move to...
* guix/utils.scm: ... here.  New procedure.
* guix/scripts/pull.scm: Use it.

guix system: Add the 'list-generations' command.

* guix/scripts/system.scm (display-system-generation, list-generations):
  New procedures.
  (process-action): Clarify docstring.
  (process-command): New procedure.
  (guix-system)[parse-sub-command]: Add 'list-generations'
  Call 'process-command' instead of 'process-action'.
* doc/guix.texi (Using the Configuration System): Mention generations,
  rollback, and 'list-generations'.
  (Invoking guix system): Document 'list-generations'.

guix system: Factorize boot parameter parsing.

* guix/scripts/system.scm (<boot-parameters>): New record type.
  (read-boot-parameters): New procedure.
  (previous-grub-entries)[system->grub-entry]: Use it.

ui: Add procedures to display a profile generation.

* guix/scripts/package.scm (guix-package)[process-query](list-generations):
  Move part of the body to 'delete-generation' and
  'display-profile-content'.
* guix/ui.scm (display-generation, display-profile-content): New
  procedures.

utils: Add 'readlink*'.

* guix/scripts/package.scm (readlink*): Move to...
* guix/utils.scm (readlink*): ... here.  New procedure.

guix system: Extract action processing.

* guix/scripts/system.scm (process-action): New procedure.  Extracted
  from...
  (guix-system): ... here.  Use it.

ui: Add 'matching-generations'.

* guix/scripts/package.scm (matching-generations): Move to...
* guix/ui.scm (matching-generations): ... here.

doc: Add a REPL example.

* doc/contributing.texi (Running Guix Before It Is Installed): Add REPL
  example.
* doc/emacs.texi (Emacs General info): Add @cindex.

gnu: Add python-file.

* gnu/packages/python.scm (python-file, python2-file): New variables.

gnu: Add RPM.

* gnu/packages/package-management.scm (rpm): New variable.
* gnu/packages/backup.scm (libarchive): Add comment.

gnu: racket: Update to 6.2.1.

* gnu/packages/scheme.scm (racket): Update to 6.2.1.