Rename thread-alive-p to thread-live-p

Rename thread-alive-p to thread-live-p

* doc/lispref/threads.texi (Basic Thread Functions): Use thread-live-p.

* etc/NEWS: 'thread-alive-p' has been renamed to 'thread-live-p'.

* src/thread.c (thread_live_p): Rename from thread_alive_p.  Adapt
all callees.
(Fthread_live_p): Rename from Fthread_alive_p.
(syms_of_threads): Make thread-alive-p an alias of thread-live-p.

* test/src/thread-tests.el (all): Replace `thread-alive-p' by
`thread-live-p'.
(threads-live): Rename from `threads-alive'.