[PATCH] Extend tcp_outgoing_address for route selection

Looking into bug 3901 if we skip past the broken assumptions and drop
half of the proposed patch...

The root cause of this report is that we have no way to selectively
per-IP determine that the outgoing connection should not be made. We
have miss_access/always_direct/never_direct acting at a gross
per-request level which way destinations should be located. But nothing
equivalent takes place after the particular destination IPs are identified.

The interesting part of the proposed bug fix is to allow
tcp_outgoing_address selection produce an invalid output which causes
the connection to be discarded from the set of upstream sources. I have
polished that up a bit and submit here. The invalid address used is
NOADDR, since ANYADDR is used for signalling system-assigned local IP.

I have also updated the documentation of tcp_outgoing_address to detail
the special values that can be configured there and what they do.

Amos