Implement a proper StringIsLocalhostIP() function

As part of https://github.com/cfengine/core/pull/4680 a naive implementation of StringIsLocalhostIP() function was introduced. It was only used as a helper recognition function in the code there so it didn't have to be bullet-proof. Nevertheless, a proper function with unit tests and also recognizing IPv6 localhost addresses would be better.

Acceptance criteria:

• a new StringIsLocalhostIP() function added to libntech/libutils
• function properly checks if the given string is a localhost IP address or not
• IPv6 localhost addresses are recognized
• the function has an extensive unit test coverage for both valid and invalid strings
• (optional) the function should only iterate over the string once (keep in mind that things like strlen() also iterate over the string)
• the function is used in cf-execd/cf-execd-runagent.c instead of the naive implementation
• manual checks are performed to see if it can be used somewhere else where we already do localhost IP address recognition/validation