Rename symbol in Hfilter

Rename symbol HFILTER_HOSTNAME_NOPTR to HFILTER_HOSTNAME_UNKNOWN
Rename symbol HFILTER_HOSTNAME_NOPTR
Update metrics.conf
Fix Hfilter for null text part

Fix HFILTER_URL_ONELINE for null text part
Tokenize subject even if a message is empty.
Fix rule MISSING_MIMEOLE for IPB forum

Add exception for Invision Power Board forum
Remove dated configuration example & add another missing option registration
Tweak default RBL module config for brevity
Fix documentation for RBL module - rbls {} was not referenced
Tweak updated config example for consistency/clarity