Check `port` options for invalid values (ports are parsed as uint16, so
in practice values >65535 are invalid; port 0 is undefined and therefore
considered invalid too). This allows for an early rejection of faulty
values and an supplying an informative message to the user.
Splits tests in `feature_proxy.py` to cover both invalid `hostname`
and `port` values.
Adds a release-note as previously valid `-port` and `-rpcport` values
can now result in errors.
