nflog: fix memory leaks

This fixes the following memory leaks:

[src/source-nflog.c:222]: (error) Memory leak: ntv
[src/source-nflog.c:236]: (error) Memory leak: ntv
[src/source-nflog.c:253]: (error) Memory leak: ntv
[src/source-nflog.c:258]: (error) Memory leak: ntv
Bug 1098: improve invalid pcre/R handling

When not using a file_data or similar 'sticky buffer', a pcre/R option
needs a content in the same buffer.
Fix libcap-ng configure typo.
Fix issue #1214

When applying wildcard thresholds (with sid = 0 and/or gid = 0) it's wrong
to exit on the first signature already having an event filter. Indeed,
doing so results in the theshold not being applied to all subsequent
signatures. Change the code in order to skip signatures with event
filters instead of breaking out of the loop.