When we support the Milan protocol, we'll use a default port. But
for now, don't listen at all unless a port is specified.
Fixes: #54
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This significantly reduces re-testing of the same paths, and simplifies
the addition of new tests.
make check -j12 times:
Before: 9m24.973s
After: 7m52.005s
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>