* libguile/threads.c (do_thread_exit): Deal with `t->mutexes' being a
weak list.
(fat_mutex_lock): Use weak-car pairs when building up `t->mutexes'.
* test-suite/tests/threads.test ("mutex-ownership")["mutex with owner
not retained (bug #27450)"]: New test.
* libguile/threads.c (scm_join_thread_timed): Recheck t->exited before
looping round to call block_self again, in case thread t has now
exited.
* test-suite/tests/threads.test ("don't hang when joined thread
terminates in SCM_TICK"): New test.
* libguile/threads.c (fat_mutex_unlock): Unblock asyncs when breaking
out of loop.
* test-suite/tests/threads.test (asyncs-still-working?): New function,
to test if asyncs are working (i.e. unblocked). Use this throughout
threads.test, in particular before and after the "timed locking
succeeds if mutex unlocked within timeout" test.
