Support --listen.

This allows the provision of address/port pairs to be listened on instead
of just providing the port. e.g.:
btcd --listen 1.2.3.4:4321 --listen 127.0.0.01 --listen [::1]:5432

When --proxy and --connect are used, we disable listening *unless* any --listen
arguments have been provided, when we will listen on those addresses as
requested.

Initial code by davec, integration by myself.

Closes #33

allow listens to fail, but warn. error if all failed

fmt