extensions: libarpt_mangle: Use guided option parser

extensions: libarpt_mangle: Use guided option parser

Sadly not the best conversion, struct arpt_mangle is not ideal for use
as storage backend: With MAC addresses, xtopt_parse_ethermac() refuses
to write into *_devaddr fields as they are larger than expected. With
XTTYPE_HOSTMASK OTOH, XTOPT_PUT is not supported in the first place.

As a side-effect, network names (from /etc/networks) are no longer
accepted. But earlier migrations to guided option parser had this
side-effect as well, so probably not a frequently used feature.

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