ps: possibility to display slice unit for a process

Library systemd-login offers possibility to display
name of a systemd slice unit for specific pid.

This patch adds output option "slice" which will
show name of systemd slice unit.

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

ps/ps.1

@@ -1582,6 +1582,10 @@ to dirty all writable pages and then be swapped out.  This number is very
 rough!
 T}
 
+slice	SLICE	T{
+displays slice unit which a process belongs to.
+T}
+
 spid	SPID	T{
 see
 .BR lwp .