ps: possibility to display seat for a process

Library systemd-login offers possibility to display
name of seat for a session on multi-seat systems.

This patch adds output option "sd_seat" which will
show name of seat or "-", when name of seat can not
be determined, but "seat0" should always exist.

To maintain compatibility with non-systemd systems,
procps must be configured with --with-systemd option
to enable this option.

ps/ps.1

@@ -1497,6 +1497,10 @@ sd_ouid	OWNER	T{
 displays the Unix user identifier of the owner of the session of a process.
 T}
 
+sd_seat	SEAT	T{
+displays login session identifier of a process.
+T}
+
 sd_session	SESSION	T{
 displays login session identifier of a process.
 T}