Rename main_thread to main_thread_id and simplify

Rename main_thread to main_thread_id and simplify

* src/emacs-module.c: Include syssignal.h, for main_thread_id.
[HAVE_PTHREAD]: Do not include pthread.h.
(main_thread): Remove.  All uses replaced by main_thread_id,
or by dwMainThreadId on NT.  Since the HAVE_PTHREAD code is now using
the main_thread_id established by sysdep.c, there is no need for a
separate copy of the main thread ID here.
(module_init): Remove.  All uses removed.
* src/sysdep.c (main_thread_id) [HAVE_PTHREAD]:
Rename from main_thread.  All uses changed.  Now extern.