rework IsLocalIP using standard library functions

[r41d]

• this time against unstable branch
• don't hardcode IPs but rather use the standard library for checking loopback and private networks
• one of the old checks with strings.HasPrefix(ip, "172.") was too broad because the private range is 172.16/12 and not 172/8
• addtional private networks are added

[cla-bot]

We require contributors to sign our Contributor License Agreement. In order for us to review and merge your code, add yourself to the .clabot file as contributor, as a way of signing the CLA.