Fixes to documentation browsing.

* +default+: Treat empty local-path as index.html (not index.xhtml).
Also, don't bother sending a 301 re-direct - just handle it.
* browse-kawa-manual: No longer use flakey frameset version.
* kawa.texi: Update url for manual.
* with-frames.html: Incomplete fix/update. Probably obsolete.
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