Avoid quitting inside a critical section on MS-Windows

Avoid quitting inside a critical section on MS-Windows

* src/w32uniscribe.c (uniscribe_list_family):
* src/w32font.c (w32font_list_family, w32font_text_extents)
(w32font_list_internal, w32font_match_internal)
(list_all_matching_fonts): Prevent quitting while these functions
cons lists of fonts, to avoid leaving the critical section taken
by the main thread, which will then cause any other thread
attempting to enter the critical section to hang.  (Bug#25279)