x11-common: Merge into xserver-nodm-init

x11-common: Merge into xserver-nodm-init

Move the (non-factual) x utils dependencies from x11-common to
x11 packagegroup.

Remove some obsolete configuration from x11-common:
 * Xsession.d/12keymap.sh: commented out xmodmap call for kdrive
 * default.xmodmap: xmodmap file used by 12keymap.sh
 * Xdefaults: rxvt configuration

At this point x11-common is just /etc/X11/Xsession and three
non-intrusive Xsession scripts: make these explicitly part of
xserver-nodm-init. RCONFLICT with the versions of xserver-common
that also provide these files.

VIRTUAL-RUNTIME_xserver_common is no longer a real abstraction but
preserve the setting for backwards compatibility (if the variable
is set to "xserver-common", the right thing still happens).

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>