proxy/tripe-mitm.c, server/tests.at: Optionally use kernel-assigned ports.

Add an option for the `peer' ports in `tripe-mitm' to be allocated by
the kernel, and for the port numbers to be written to named files.  Use
this in the tests so as to avoid conflicts between concurrent instances
of the tests.  It's not perfect: some of the tests require that a new
`tripe-mitm' take over a port from an old one, and there's a chance that
some other process might have grabbed it in between -- but it's much
better than it was before.

I should also consider using `noip' for this testing.