5008409b5c introduced the usage of
`strings.Cut` to help parse listener addresses.
Part of that also made it error out if no addr is specified after the
protocol spec (e.g. `tcp://`).
Before the change a proto spec without an address just used the default
address for that proto.
e.g. `tcp://` would be `tcp://127.0.0.1:2375`, `unix://` would be
`unix:///var/run/docker.sock`.
Critically, socket activation (`fd://`) never has an address.
This change brings back the old behavior but keeps the usage of
`strings.Cut`.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
