An hostport of "[::1]" now results in the same error message
"missing port in address" as the hostport value "127.0.0.1",
so SplitHostPort won't complain about "too many colons
in address" anymore for an IPv6 address missing a port.

Added tests checking the error values.

Fixes #4526.

R=dave, rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7038045