smtp v17.3

PR https://github.com/inliniac/suricata/pull/1189 with additional fixes, now passes my QA.

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/48
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/48