http: add meta-field-limit option

The meta-field-option allows for setting the hard limit of request
and response fields in HTTP. In requests this applies to the request
line and headers, not the body. In responses, this applies to the
response line and headers, not the body.

Libhtp uses a default limit of 18k. If this is reached an event is
raised.

Ticket 986.

https://redmine.openinfosecfoundation.org/issues/986
https://buildbot.suricata-ids.org/builders/inliniac/builds/55

changes since https://github.com/inliniac/suricata/pull/648
- reworded the commit message