The argument of the first parameter for connection setup functions on
IP networks must contain a protocol name or number. This change adds
validation for arguments of IP networks to connection setup functions.

Fixes #18185.

Change-Id: I6aaedd7806e3ed1043d4b1c834024f350b99361d
Reviewed-on: https://go-review.googlesource.com/40512
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>