Rename primary_thread to main_thread

Rename primary_thread to main_thread

This avoids the confusion of using two different phrases "main thread"
and "primary thread" internally to mean the same thing.  See:
http://lists.gnu.org/archive/html/emacs-devel/2016-12/msg01142.html
* src/thread.c (main_thread): Rename from primary_thread,
since the new name no longer clashes with main_thread_id
and Emacs internals normally call this the "main thread".
(init_main_thread): Rename from init_primary_thread.
(main_thread_p): Rename from primary_thread_p.
All uses changed.