Fix try-lock-thread-mutex to work when there is no current thread.

Fix try-lock-thread-mutex to work when there is no current thread.

GC daemons need to run when there is no current thread.  Try-lock-
thread-mutex was intended for their use.  They do not mind
occasionally skipping an opportunity to clean, including the rare
times when a GC interrupt occurs with no current thread -- in the
thread system of a quiescent world.

Try-lock-thread-mutex was signaling spurious deadlocks when GC daemons
ran with no current thread.  Free locks appeared to be owned by #F --
the current thread.