Use netmasks instead of prefix lengths internally

Use netmasks instead of prefix lengths internally

Iptables supports using non-continuous netmasks like FFFF::FFFF which would
match the first and last 16bit of an IPv6 address while ignoring the parts
in between which is useful fordeclaring rules targeting hosts on rotating
prefixes.

Instead of storing parsed netmasks as bitcount internally, use a full mask
which is passed to iptables as-is.

Also support a new shorthand notation "addr/-N" which will construct a mask
that matches the *last* N bits of an address - useful for matching the host
part only of an IPv4 address, e.g.

  option dest_ip '::c23f:eff:fe7a:a094/-64'

This will convert to a netmask of "::ffff:ffff:ffff:ffff".

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>