Tidy up some stray / unused stuff in the IPFW package and friends.

• unifdef -DCOMPAT_IPFW (this was on by default already)
• remove traces of in-kernel ip_nat package, it was never committed.
• Make IPFW and DUMMYNET initialize themselves rather than depend on compiled-in hooks in ip_init(). This means they initialize the same way both in-kernel and as kld modules. (IPFW initializes now :-)