rtsock changes for multiple fibs support

[Alexander V. Chernikov]

Hello list!

At the moment multiple fib support is completely broken in route(4)
(described in kern/134931)

Linux supports fibs via its netlink protocol. OpenBSD makes rtsock 
version bump for the same for a while ago.

Since 9.0 is approaching it is a good time to make some changes to
routing socket:

Goals:
* Permit fib to be get/set via route(4) interface
* Keep existing ABI/API as much as possible
* Permit various route(4) headers changes without breaking ABI

Proposed structure changes:
add

uint16_t fib
uint16_t reserved

to rt_msghdr, if_msghdr, ifa_msghdr, ifma_msghdr, if_announcemsghdr

Proposed changes to undocumented sysctl route dump interface:
* Add (optional) .FIB.RTSOCK_VERSION to sysctl
net.route.0.ADDRESS_FAMILY.NET_RT_DUMP.FLAGS oid:

* document such interface in route(4)

Proposed ABI keeping techniques:

RTSOCK:

1) Add SO_RTSOCKVERSION to SOL_SOCKET setsockopt level
2) Determine rtsock version via __FreeBSD_version checking
3) Add net.rtsock.force_version sysctl
4) Do __FreeBSD_version && rtsock version bump

Rtsock version selection algorithm:
if
SO_RTSOCKVERSION is set on socket: => use selected version (or return -1 in
setsockopt)
else if
net.rtsock.force_version > 0 => use forced version
else
net.rtsock.force_version == 0 => use version based on __FreeBSD_version
check

Examples:

"New" binaries (build with new headers):
* have __FreeBSD_version >= X so "current" rtsock version is used

"Old" binaries (from 7.X/8.X etc..):
* have __FreeBSD_version < X so "previous" rtsock version is used

Old/New binaries with New/Old libraries using rtsock:
This is the most tricky part.

Major routing software (quagga, openbgpd, bird) do not use libraries
for rtsock interaction, however such libraries exists (libpdel, for example)

What can be done:
(required) Add explicit rtsock version request vie SO_RTSOCKVERSION in 
all FreeBSD
base code (route(8), netstat(8), ...)

(optional) Add patches for SO_RTSOCKVERSION to ported versions/upstream 
versions of
libpdel (and possibly for routing daemons)

If someone wants to use newly-compiled library (or binary) for which
__FreeBSD_version check happens incorrectly he can explicitly set
net.rtsock.force_version to needed value. This will not break all
software aware of SO_RTSOCKVERSION (base binaries).

Those measures can't cover all 100 percent of users, but maybe 99 is
enough? Anyway, rebuilding world/userland with this patch should not 
break anything. Problems can arise only when some recompilation based on 
new headers is done.


SYSCTL ROUTE DUMP:
SO_RTSOCKVERSION is replaced by more detailed sysctl request described 
in the beginning, the rest approach is exactly the same (patches for 
base/ports, etc..)



If this is more or less reasonable approach I can provide patch doing
all of the above till the end of this week