ipfw - Fix broken mixed network and host IP specifications in ip tables

ipfw - Fix broken mixed network and host IP specifications in ip tables

* ipfw improperly assumes that the netmask sin_len is pre-zero'd, but
  prior table entries on the same command line which specify a network
  mask will leave the field non-zero for later host entries also specified
  on the command line:

        ipfw table 1 add 10.0.0.0/8 192.0.2.1     # 2^24 + 1 addresses

	# ipfw table 1 print
	10.0.0.0/8
	192.0.0.0/8	<--- wrong

* Fix the issue by properly initializing netmask.sin_len to 0 when a
  host IP is specified.

Submitted-by: Martin Neitzel <neitzel@marshlabs.gaertner.de>