This allows serials containing colons to sneak through the PuTTY hostname
mangling code; it thinks they're ipv6 addresses, so doesn't go and strip
and discard the port.
This is useful for adbwireless, which generates serials of the form
"ip:port", e.g. "192.168.0.2:5555". Now ":192.168.0.2:5555" is valid.