gnu: gnome: Depend on xdg-user-dirs.

* gnu/packages/gnome.scm (gnome): Add xdg-user-dirs dependency.  The presence
  of the autostart file in /run/current-system/profile/etc/xdg should cause
  xdg-user-dirs to be run when a user logs in, ensuring that they have
  ~/Pictures and other well-known user directories.

gnu: Add xdg-user-dirs.

* gnu/packages/freedesktop.scm (xdg-user-dirs): New public variable.

gnu: Add radare2.

* gnu/packages/engineering.scm (radare2): New variable.

gnu: Add python-capstone.

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

gnu: Add capstone.

* gnu/packages/engineering.scm (capstone): New variable.

gnu: youtube-dl: Update to 2017.08.23.

* gnu/packages/video.scm (youtube-dl): Update to 2017.08.23.

Signed-off-by: Arun Isaac <arunisaac@systemreboot.net>

gnu: pv: Update to 1.6.6.

* gnu/packages/pv.scm (pv): Update to 1.6.6.

gnu: linux-libre: Update to 4.12.9.

* gnu/packages/linux.scm (%linux-libre-version): Update to 4.12.9.
(%linux-libre-hash): Update hash.

gnu: linux-libre@4.9: Update to 4.9.45.

* gnu/packages/linux.scm (linux-libre-4.9): Update to 4.9.45.

gnu: linux-libre@4.4: Update to 4.4.84.

* gnu/packages/linux.scm (linux-libre-4.4): Update to 4.4.84.

gnu: go: Update to 1.9.

* gnu/packages/golang.scm (go-1.8): Replace with ...
(go-1.9): ... new variable.
[arguments]: Adjust the pre-build phase accordingly.

gnu: gdm: Fix environment of worker processes and sessions.

* gnu/packages/gnome.scm (gdm): Get X server path from GDM_X_SERVER
environment variable, and make sure this variable is propagated to internal
sub-processes.  Launch sessions with a reasonable XDG_CONFIG_DIRS setting.
Get the custom GDM conf from the current system's profile.

gnu: gdm: Use proper elogind check, and wrap environment.

* gnu/packages/gnome.scm (gdm): Use glib-or-gtk-build-system to make sure the
schemas are available to GDM.  Install gdm to /bin to make this happen.  Use a
proper elogind check.

gnu: gdm: Look for sessions in /run/current-system.

* gnu/packages/gnome.scm (gdm): Fix search paths for system sessions.  Use
  /var as localstatedir.  Set a more sensible default path for user sessions.

gnu: accountsservice: Properly reference shadow.

* gnu/packages/freedesktop.scm (accountsservice): Fix references to binaries
  provided by the shadow package.

gnu: parallel: Update to 20170822.

* gnu/packages/parallel.scm (parallel): Update to 20170822.

gnu: yoshimi: Update to 1.5.3.

* gnu/packages/music.scm (yoshimi): Update to 1.5.3.

gnu: zynaddsubfx: Update to 3.0.2.

* gnu/packages/music.scm (zynaddsubfx): Update to 3.0.2.

gnu: guile-xcb: Update source URL.

* gnu/packages/guile-wm.scm (guile-xcb)[source]: Use archived tarball URL.

gnu: font-mathjax: Move to javascript.scm.

This avoids cross-module top-level references.

* gnu/packages/fonts.scm (font-mathjax): Move to...
* gnu/packages/javascript.scm (font-mathjax): ... here.  New variable.