Rename font_driver member close -> close_font

Rename font_driver member close -> close_font

* src/alloc.c (cleanup_vector):
* src/xftfont.c (xftfont_driver):
* src/xfont.c (xfont_driver):
* src/nsfont.m (nsfont_driver):
* src/macfont.m (macfont_driver):
* src/ftxfont.c (ftxfont_driver):
* src/ftfont.c (ftfont_driver):
* src/ftcrfont.c (ftcrfont_driver):
* src/font.h (struct font_driver):
* src/font.c (font_clear_cache, font_close_object):
Rename `close' member to `close_font', to avoid clash with preprocessor
define of `close' in nt/inc/ms-w32.h and for consistency with `open_font'.