NO_PROXY includes support for CIDR, and notations can also
match exactly on port information if provided.
When specifying a port with IPv6, the address must be enclosed with
square brackets, [IPv6 address]:port.

Updates golang/go#16704 (fixes after vendor into std)

Change-Id: Ideb61a9ec60a6b1908f5a2c885cd6d9dd10c37cf
Reviewed-on: https://go-review.googlesource.com/115255Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>