MFC: Make ip6fw as loadable module.

netinet6/ip6_forward.c: 1.9 -> 1.10
netinet6/ip6_fw.c: 1.6 -> 1.7
netinet6/ip6_fw.h: 1.6 -> 1.7

	netinet6/ip6_input.c:	1.15 -> 1.16

netinet6/ip6_output.c: 1.17 -> 1.18
modules/Makefile: 1.139 -> 1.140