Library systemd-login offers possibility to display
name of systemd user unit for specific pid. Note that not all
processes are part of a user unit.
This patch adds output option "sd_uunit" which will
show name of user unit or "-", when process does not belong
to any user unit. This is similar to "sd_unit" but applies
to user units instead of system units.
To maintain compatibility with non-systemd systems,
procps must be configured with --with-systemd option
to enable this option.