ps: possibility to display login session for a process

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.