filestore: fix parsing bug

Filestore keyword can have options or no options, and the parser
was enforcing the NOOPT flag too strictly.

Bug #1288 https://redmine.openinfosecfoundation.org/issues/1288

Prscript:
- PR build: https://buildbot.openinfosecfoundation.org/builders/inliniac/builds/25
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/inliniac-pcap/builds/25