Pull up following revision(s) (requested by knakahara in ticket #462):
sys/net/if_gif.c: revision 1.133, 1.134, 1.137
sys/net/if_gif.h: revision 1.28-1.29
sys/netinet/in_gif.c: revision 1.90-1.91
sys/netinet/in_gif.h: revision 1.18
sys/netinet6/in6_gif.c: revision 1.88-1.89
sys/netinet6/in6_gif.h: revision 1.17
preserve gif(4) configs by psref(9) like vlan(4) and l2tp(4).
After Tx side does not use softint, gif(4) can use psref(9) for config
preservation like vlan(4) and l2tp(4).
update locking notes later.
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update gif(4) locking notes.
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IFF_RUNNING checking in Rx and Tx processing is unnecessary now.
Because the configs of gif (members of gif_var) are protected by psref(9).
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remove duplicated null ckeck

(snj)