net: add IP.IsPrivate

Adds a new method IsPrivate to check if an IP is
private according to RFC 1918 & RFC 4193.

Fixes #29146

Change-Id: If77b9e1746d86029df66ae9f18437b1f65a18b59
GitHub-Last-Rev: 09f4ba75988c87b90550d071866ab1d6634ef681
GitHub-Pull-Request: golang/go#42793
Reviewed-on: https://go-review.googlesource.com/c/go/+/272668


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>