user, xsession: implement a correct way to stop the graphic session.

The "isolate overall.target" method caused havoc on system shutdown,
conflicting with systemd's attempt to stop the user instance altogether
("start exit.target").

The new way introduces a special "exit-session@.target" which is made
conflicting with each per-session unit via need-session@ templates. Then
it's enough just to start this target to stop all session-related units.
xsession/Xsession: fix handling of session string.