ps: possibility to display uid of session owner for a process

ps: possibility to display uid of session owner for a process

Library systemd-login offers possibility to display the Unix
user identifier of the owner of the session of a process.
This information will also be displayed for user processes which
are shared between multiple login sessions of the same user,
where sd_session will be blank.

This patch adds output option "sd_ouid" which will show
user UID or "-", when there is no owner for a process.

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