Remove pr_destroy in favour of VNET_SYSUNINITs.

[bz]

Summary

The pr_destroy field does not allow us to run the teardown code
in a specific order. VNET_SYSUNINITs however are doing exactly that.
Thus remove the VIMAGE conditional field from the domain(9) protosw
structure and replace it with VNET_SYSUNINITs.
This also allows us to change some order and to make the teardown functions
file local static.
Also convert divert(4) as it uses the same mechanism ip(4) and ip6(4) use
internally.

Slightly reshuffle the SI_SUB_* fields in kernel.h and add a new ones, e.g.,
for pfil consumers (firewalls), partially for this commit and for others
to come.

[bz]

Add Michael for the SCTP change.

[tuexen]

The SCTP changes are fine, therefore the 'Accept Revision' for them.
Once this hits the svn tree, I'll integrate the changes upstream.

[jhb]

sys/sys/kernel.h

142

s/Intialize/Initialize/.

147

Maybe SI_SUB_FIREWALL? FW is perhaps ambiguous (e.g. "firmware") OTOH, given it's in the middle of SI_SUB_PROTO_* perhaps SI_SUB_PROTO_FW? (I think it would be clearer if we did 's/SI_SUB_PROTO_/SI_SUB_NET_/' but that's a separate topic.)

[bz]

Addressed the changes suggested by @jhb . Thanks.