[libextractor] return of getKeywords()

[Ryan Underwood]

Hello,

It seems reasonable that getKeywords should have a different return type
in the two cases where:
1) A file was handled, but had no keywords
2) No plugin could handle the file.

The distinction is important for certain programs (which are indexing
metadata based on file extensions, or on libmagic determine MIME type).
In the first case, the program using libextractor would continue to try
to extract data for that file type.  In the second case, the program can
ignore all future files of that type, resulting in a significant
speedup.

The plugin interface will need to be modified to instead of returning
the keyword list which is modified through pointer anyway, return
HANDLED or NOT_HANDLED.  Then if nobody handled, propagate that
condition to the library caller somehow.

-- 
Ryan Underwood, <address@hidden>