Use a different mutex to protect the kqueue klist in logsoftc.

Use a different mutex to protect the kqueue klist in logsoftc.

knote_locked() will call wakeup() and with it the SCHED_LOCK and by that
makes log_mtx no longer a leaf lock. By using an own lock for the klist
we can keep log_mtx a leaf lock and with that printf(9) can be used in
most contexts again.
OK mvs@