Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Various cleanup done in ipfw3-head branch including:
- use a uniform mtag format for all packets that exit and re-enter
the firewall in the middle of a rulechain. On reentry, all tags
containing reinject info are renamed to MTAG_IPFW_RULE so the
processing is simpler.

- make ipfw and dummynet use ip_len and ip_off in network format
everywhere. Conversion is done only once instead of tracking
the format in every place.

- use a macro FREE_PKT to dispose of mbufs. This eases portability.

On passing i also removed a few typos, staticise or localise variables,
remove useless declarations and other minor things.

Overall the code shrinks a bit and is hopefully more readable.

I have tested functionality for all but ng_ipfw and if_bridge/if_ethersubr.
For ng_ipfw i am actually waiting for feedback from glebius@ because
we might have some small changes to make.
For if_bridge and if_ethersubr feedback would be welcome
(there are still some redundant parts in these two modules that
I would like to remove, but first i need to check functionality).

Introduce a div_destroy() function which takes over per-vnet cleanup tasks
from the existing modevent / MOD_UNLOAD handler, and register div_destroy()
in protosw as per-vnet .pr_destroy() handler for options VIMAGE builds. In
nooptions VIMAGE builds, div_destroy() will be invoked from the modevent
handler, resulting in effectively identical operation as it was prior this
change. div_destroy() also tears down hashtables used by ipdivert, which
were previously left behind on ipdivert kldunloads.

For options VIMAGE builds only, temporarily disable kldunloading of ipdivert,
because without introducing additional locking logic it is impossible to
atomically check whether all ipdivert instances in all vnets are idle, and
proceed with cleanup without opening a race window for a vnet to open an
ipdivert socket while ipdivert tear-down is in progress.

While here, staticize div_init(), because it is not used outside of
ip_divert.c.

In cooperation with: julian
Approved by: re (rwatson), julian (mentor)
MFC after: 3 days

Convert IPDIVERT into a loadable module. This makes use of the dynamic loadability
of protocols. The call to divert_packet() is done through a function pointer. All
semantics of IPDIVERT remain intact. If IPDIVERT is not loaded ipfw will refuse to
install divert rules and natd will complain about 'protocol not supported'. Once
it is loaded both will work and accept rules and open the divert socket. The module
can only be unloaded if no divert sockets are open. It does not close any divert
sockets when an unload is requested but will return EBUSY instead.

Re-remove MT_TAGs. The problems with dummynet have been fixed now.

Tested by: -current, bms(mentor), me
Approved by: bms(mentor), sam