suricata: ignore SIGHUP signal

This patch ignores the SIGHUP signal instead of having the default
behavior.

Ticket: https://redmine.openinfosecfoundation.org/issues/911

Builds:
- PR build: https://buildbot.suricata-ids.org/builders/regit/builds/73
- PR pcaps: https://buildbot.suricata-ids.org/builders/regit-pcap/builds/11