config: add udev/systemd multi-seat support

Add support for multi-seat-aware input device hotplugging. This
implements the multi-seat scheme explained here:

http://www.freedesktop.org/wiki/Software/systemd/multiseat

This introduces a new X server switch "-seat" which allows configuration
of the seat to enumerate hotplugging devices on. If specified the value
of this parameter will also be exported as root window property
Xorg_Seat.

To properly support input hotplugging devices need to be tagged in udev
according to the seat they are on. Untagged devices are assumed to be on
the default seat "seat0". If no "-seat" parameter is passed only devices
on "seat0" are used. This means that the new scheme is perfectly
compatible with existing setups which have no tagged input devices.

Note that the -seat switch takes a completely generic identifier, and
that it has no effect on non-Linux systems. In fact, on other OSes a
completely different identifier scheme for seats could be used but still
be exposed with the Xorg_Seat and -seat.

I tried to follow the coding style of the surrounding code blocks if
there was any one could follow.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

man/Xserver.man

@@ -220,6 +220,12 @@ sets screen-saver timeout time in minutes.
 .B \-su
 disables save under support on all screens.
 .TP 8
+.B \-seat \fIseat\fP
+seat to run on. Takes a string identifying a seat in a platform
+specific syntax. On platforms which support this feature this may be
+used to limit the server to expose only a specific subset of devices
+connected to the system.
+.TP 8
 .B \-t \fInumber\fP
 sets pointer acceleration threshold in pixels (i.e. after how many pixels
 pointer acceleration should take effect).