Rename font_driver member open -> open_font

Rename font_driver member open -> open_font

* 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_open_entity):
Rename `open' member to `open_font', to avoid clash with preprocessor
define of `open' in lib/fcntl.h and nt/inc/ms-w32.h.  Remove earlier
#undef hack.