libxtables: have xtopt_parse_mint interpret partially-spec'd ranges

libxtables: have xtopt_parse_mint interpret partially-spec'd ranges

When ":n" or "n:" is specified, it will now be interpreted as "0:n"
and "n:<max>", respecitvely. nvals will always reflect the number of
(expanded) components. This restores the functionality of options that
take such partially-unspecified ranges.

This makes it possible to nuke the per-matchdata init functions of
some extensions and simply the extensions postparsing to the point
where it only needs to check for nvals==1 or ==2.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>