Implement support to rsub operator into libmodsecurity

[zimmerle]

Operator rsub needs to be implemented inside libmodsecurity.

The documentation of the operator can be found here:
https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual#rsub

The file already exist in the repository:
https://github.com/SpiderLabs/ModSecurity/blob/libmodsecurity/src/operators/rsub.cc

[zimmerle]

As content injection is not supported on v3, marking this as deprecated.