mount.cifs: clean up option parsing

mount.cifs: clean up option parsing

Moved option string parsing to function parse_opt_token(char*).  Main
loop in parse_options(const char*, struct parsed_mount_info*)
transplanted to a switch block.

The parsing function folds common options to a single macro:
	1.) 'unc','target', and 'path' -> 'OPT_UNC'
	2.) 'dom*' and 'workg*' -> 'OPT_DOM'
	3.) 'nobrl' and 'nolock' -> 'OPT_NO_LOCK'

Kept 'fmask' and 'dmask' (OPT_FMASK, OPT_DMASK), which fall through to
'file_mode' and 'dir_mode' in the main loop.

Signed-off-by: Scott Lovenberg <scott.lovenberg@gmail.com>