Add new 'user-session-launch' service

This patch series adds a new service to 'user-session-units' that starts a
graphical systemd-logind session, which kicks off user@UID.service. A
command-line option is mandatory, and the service starts a user session for
that user on a given seat.

It reuses some code from uxlaunch (pam.c), and launcher.c is new code.