CFR: ipfw locking

[Sam Leffler]

http://www.freebsd.org/~sam/ipfw.patch

These changes add locking.  There are two locks: one for static rules and 
one for dynamic rules.  The associated dummynet changes are not included as 
I haven't tested them yet.  The locking scheme may need to be revised to 
use something like sx locks; this will be revisited in a later pass.

I've been running with these changes for a week on my interior firewall, 
but they are very lightly tested and need a good review and more testing. 
I don't plan to commit them until they get more of each.

	Sam