xap_helper: enable FLAG_PURE_NOT in external process

xap_helper: enable FLAG_PURE_NOT in external process

Since public-facing WWW uses an external process with
a non-blocking socket, clients will only see 503 errors
if the search is overloaded.  While allowing pure NOT
queries is expensive, there are already many possible
ways of triggering expensive queries so it's probably not
a problem to allow one more.