receptor: fixup spaces/tabs mess
relay_hostname: increase to max length of 255

It seems the length returned by gethostname is kind of messy.  SUSv1
seems to limit this to 255 bytes.  Linux limits it to 64 bytes (so our
128 were already too many there.)  A define HOST_NAME_MAX may be
available, POSIX doesn't define a constant for it, so yay.  Let's
increase the array to 256, to support the 255 bytes len.
relay: add option to listen on a specific interface

This is based on commit 6989e8f938a1f14b6af14f50ea442c7a79cbbf9c by Jose
Riguera <jriguera@gmail.com>.

I've cleaned it up and made sure we still support IPv6 by default (e.g.
don't change existing behaviour if we don't specify -i on command line).