[MERGE] Set SO_REUSEADDR on server sockets (#164288)

[Andrew Bennetts]

Martin Pool wrote:
> AfC reported that sockets left behind in CLOSE_WAIT make starting bzr serve problematic.
> 
> I can't reproduce the problem in brief ad-hoc testing but it is a good idea to set this option from server processes, so here it is anyhow.  I think a functional test for it would be unduly complicated (and as I say I can't reproduce it on localhost.)
> 
> https://bugs.edge.launchpad.net/bzr/+bug/164288

bb:approve

-Andrew.