extended search fix

This patch fixes a bug in the search code and did some
small optimizations.

The possibility to start a search with _only_ excluded
words (only words start with a minus sign) was disabled
because this seem to crash PHP 4.3.10 (at least the ppc
version :-()

darcs-hash:20050604194621-7ef76-10a5bb6a5b2804bcadcbffcdf2ea500db7ceeb91.gz