add FTS3 _backward_compatibility mode to let apps use FTS1 or 2.

bug:2651952
this mode was previously available in Eclair and below. this mode was removed
by me for no particular reason in Froyo.
this change doesn't bring in any new modules into sqlite. just
allows FTS1 or 2 apps be used with FTS3 running in backward compatibility mode

Change-Id: Id5d72b440a378dd6d6703ce99b5d3d70ffd0c80f