ps: possibility to display machine name for a process

Library systemd-login offers possibility to display
the name of the VM or container which process belongs to.

This patch adds output option "sd_machine" which will
show machine name or "-" when the name can not be determined.

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_machine	MACHINE	T{
+displays machine name for processes assigned to VM or container.
+T}
+
 sd_ouid	OWNER	T{
 displays the Unix user identifier of the owner of the session of a process.
 T}