macterm.c (backtranslate_modified_keycode): New function to

macterm.c (backtranslate_modified_keycode): New function to
	mask modifiers off of keycodes for use in modifier mapping.
	(mac_determine_quit_char_modifiers): Remove
	mac-reverse-ctrl-meta and replace it with more flexible system
	mapping mac modifier keys to emacs modifier keys.
	(convert_fn_keycode): Map Fn-keys to their original keycode
	using a table (english keyboard only).
	(syms_of_macterm): Define mac-control-modifier,
	mac-command-modifier, mac-function-modifier.
	Define meta and ctrl for use as modifiers.