lei_xsearch: avoid false-positives on externals w/ L: and kw:

lei_xsearch: avoid false-positives on externals w/ L: and kw:

We need to use LeiSearch->qparse_new to handle (and filter out)
"L:" and "kw:" search prefixes to avoid hitting false positives
when externals are involved.  Unfortunately, this doesn't work
for remote HTTP(S) externals, but those aren't enabled by
default.