ps: possibility to display systemd unit for a process

ps: possibility to display systemd unit for a process

Library systemd-login offers possibility to display
name of a systemd unit file for specific pid. Note that
not all processes are part of a system unit/service
(e.g. user processes, or kernel threads).

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

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