Since Dial(":80") was implemented on Plan 9 (CL 32593),
TestProtocolDialError is failing because dialing a port
superior to 65535 is supported on Plan 9.

This change disallows dialing and listening on ports
superior to 65535.

Fixes #17761.

Change-Id: I95e8a163eeacf1ccd8ece7b650f16a0531c59709
Reviewed-on: https://go-review.googlesource.com/32594Reviewed-by: Russ Cox <rsc@golang.org>