ps: possibility to display login session for a process

Library systemd-login offers possibility to display name
of login session for specific pid.
Note that not all processes are part of a login session
(e.g. system service processes, user processes that are shared
between multiple sessions of the same user, or kernel threads).

This patch adds output option "sd_session" which will
show name of session or "-", when process does not belong
to any session.

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

ps/ps.1

@@ -1489,6 +1489,10 @@ SCHED_FIFO, SCHED_RR, SCHED_BATCH, SCHED_ISO, and SCHED_IDLE are respectively
 displayed as 0, 1, 2, 3, 4, and 5.
 T}
 
+sd_session	SESSION	T{
+displays login session identifier of a process.
+T}
+
 sd_unit	UNIT	T{
 displays systemd unit which a process belongs to.
 T}