Fulltext search: Greatly improve by using a more fuzzy approach

Fulltext search: Greatly improve by using a more fuzzy approach

Most of this done on suggestion of Enrico Zini <enrico@enricozini.org>

Using OP_OR instead of OP_AND as default can actually lead to better
matches because the ones found with OP_AND often don't actually are
the most relevant ones. This is especially true when using more than
two keywords.

Accordingly sort by relevance on the result page.

Improve indexing:
 + Add both the unstemmed and the stemmed description to index
   this will increase the relevance of exact matches. Only the
   latter is done with positional information
 + Really index debtags and the package name