This is a rework of f9059d4e "Key all browser
plugin features to --enable-nsplugin."  The problem with that was that Mac OS X
supports pluging browser plugins into LO documents (which was originally
controlled by --enable-mozilla) but not plugging LO into browser windows (which
was originally controlled by --enable-nsplugin), so controlling the former with
the same switch as the latter did not actually work.

Thus I replaced the single ENABLE_NSPLUGIN feature flag with two dedicated ones,
ENABLE_NPAPI_FROM_BROWSER (for plugging browser plugins into LO documents) and
ENABLE_NPAPI_INTO_BROWSER (for plugging LO into browser windows).  The
--enable-nsplugin configure switch is gone completely, setting the feature flags
is always done fully automatically now.

Change-Id: Iecf706637465e865c987563b5de489fa90b4c904