parse: Silence discard-const warning on OpenBSD

parse: Silence discard-const warning on OpenBSD

This might be overkill just to silence the warnings, but having
a common function for const and non-const version makes it discard
const pointer at some point.

--
    CC parse.o
parse.c: In function 'find_option_c':
parse.c:1051: warning: passing argument 1 of 'find_option' discards qualifiers from pointer target type
parse.c: In function 'get_option':
parse.c:1051: warning: passing argument 1 of 'find_option' discards qualifiers from pointer target type
parse.c:1051: warning: passing argument 1 of 'find_option' discards qualifiers from pointer target type
parse.c: In function 'parse_cmd_option':
parse.c:1051: warning: passing argument 1 of 'find_option' discards qualifiers from pointer target type

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>