Parse fwsnort rules for 'msg' fields

Parse fwsnort rules for 'msg' fields

Added the ability to acquire Snort rule 'msg' fields from fwsnort if
it's also installed.  A new variable FWSNORT_RULES_DIR tells psad where
to look for the fwsnort rule set.  This fixes a problem reported by Pui
Edylie to the psad mailing list where fwsnort logged an attack that psad
could not map back to a descriptive 'msg' field.