pam-systemd: disconnect bus connection when leaving session hook, even on error

pam-systemd: disconnect bus connection when leaving session hook, even on error

This adds support for systematically destroying connections in
pam_sm_session_open() even on failure, so that under no circumstances
unserved dbus connection are around while the invoking process waits for
the session to end.  Previously we'd only do this on success, now do it
in all cases.

This matters since so far we suggested people hook pam_systemd into
their pam stacks prefixed with "-", so that login proceeds even if
pam_systemd fails. This however means that in an error case our
cached connection doesn't get disconnected even if the session then is
invoked. This fixes that.