xshared: Introduce xt_cmd_parse_ops::option_name

xshared: Introduce xt_cmd_parse_ops::option_name

The old opt2char() function was flawed: Since not every field in
optflags contains a printable character, typical use of its return value
in print statements could lead to garbage on screen.

Replace this by a mechanism to retrieve an option's long name which
supports family-specific overrides. and get rid of optflags field
altogether and define NUMBER_OF_OPT similar to NUMBER_OF_CMD.

Signed-off-by: Phil Sutter <phil@nwl.cc>