MFC r261193: login: Clean up PAM and audit, then exit, on SIGHUP and SIGTERM

This avoids leaving stale entries in utmpx after the connection is closed on
an open login session. It also allows a clean way (SIGTERM) to forcibly
terminate a user's terminal session.

This does not affect the situation for "hung" processes after the connection
is closed. The foreground process group receives SIGHUP and the tty becomes
inaccessible.

Also replace all use of the obsolete signal() function with sigaction() (not
only the part where it is actually required: SIGHUP and SIGTERM must mask
the other as well when caught).

PR: misc/183495