Implement support to the GLOBAL, IP, SDBM_DELETE_ERROR and SESSION variables in libmodsecurity

[zimmerle]

Those variables are related to collections which are not implemented yet.

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

[zimmerlezen]

GLOBAL, IP are already ready. SESSION was moved to another ticket. SDBM_DELETE_ERROR is no longer necessary.