thread-join returns the result of finished thread

thread-join returns the result of finished thread

* doc/lispref/threads.texi (Basic Thread Functions):
* etc/NEWS: Document return value of `thread-join'.

* src/thread.c (invoke_thread_function, Fmake_thread)
(init_main_thread): Set result.
(Fthread_join): Propagate signals, and return result.
(Vmain_thread): New defvar.

* src/thread.h (struct thread_state): Add `result' field.

* test/src/thread-tests.el (threads-join): Test also return value.
(threads-join-error): New test.
(threads-mutex-signal): Check for propagation of `quit' signal.