These are the new files required by the stix smtp indicator detection
features.
Integrates the SMTP indicators detection logic into suricata.
This adds important functionality required to receive stix indicators
from yeti.
Upon inspection I discovered that the ipv6 mutex was being used when
modifying the ipv4 watchlist.
Cleaned out some compiler warnings.  Just keeping things tidy!
Adding a readme file that tells how to build/test the stix add-ons.
Conforming to OISF standards.  Codying style modifications only.