[multipathd] signal deadlock

[multipathd] signal deadlock

If multipathd is run with -v3, both the SIGHUP, and the SIGUSR1 signal handlers
will log a message.  If a multipathd thread receives one of these signals while
it has a log lock held, it deadlocks itself. Also, the SIGHUP handler will grab
the vecs lock, so if any thread receives a SIGHUP while holding the vecs lock,
it deadlocks itself.  This commit blocks the appropriate signals to guard
against this.