Implement support to variable USERAGENT_IP inside libmodsecurity

[zimmerle]

More information about USERAGENT_IP can be found at ModSecurity reference manual: https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual

[victorhora]

This feature is tied to an specific Apache module as such it belongs to the Apache connector and can't be implemented on libModSecurity. Marking as won't implement.