Replace `isip()` code with the free code from Paul Vixie

[rockdaboot]

This would allow to drop a runtime dependency on Windows.

The code is e.g. at https://opensource.apple.com/source/xnu/xnu-4903.221.2/libkern/net/inet_pton.c.auto.html

We can simplify or throw out parts of it as we are only interested in a boolean return value.