VIP15: Specifying source address for outgoing sockets

Synopsis

Make it possible to specify source address for outgoing TCP connections (backends, -M)

Why?

Traffic engineering.

How?

This is the hard bit. Ideally the IETF had paid more attention to how one specifies network addresses, but as the long list of RFC's on how to write IPv6 addresses document, they are morons.

The best proposal I can come up with is something like:

IP [PORT] '/' IP [PORT]

The trivial way to implement this would be to double the size of struct suckaddr which has merits as it could then be used to have all the info about a TCP connection, but it is a major API breakage fest.

A less intrusive way is to special-case it for -M and backend definitions, which "only" breaks that backend API for VMODS.