detect: allow alias registration for rule keywords

This allows for registering a keyword under another name while keeping
the old name active and supported.

Do this for 'luajit', which can now also be used as just 'lua'.

Prscript:
- PR build: https://buildbot.suricata-ids.org/builders/inliniac/builds/233
- PR pcaps: https://buildbot.suricata-ids.org/builders/inliniac-pcap/builds/153