bsd_tcpip cleanup and sysctl support

[Andrew Lunn <andrew dot lunn at ascom dot ch>]

Hi Folks

Attached is a rather large patch which does two things:

1) It completes the cleanup of the FreeBSD networks stack that Nick
   started. There are no more compiler warnings now for all the
   combinations of options i tried, including IPv6.

2) I added back support for sysctl. I want this for a fresh port of
   the net-snmp code. There is no new code, i just put back the code
   which was taken out when the port of the FreeBSD stack was made to
   eCos. So there is little risk with this. There is CDL to control
   this new code so it will not bloat existing applications.

Comments please and if i don't receive any i will commit the patch in
a couple of days.

  Thanks
        Andrew

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