Convert tsginidx.c's GIN indexing logic to fully ternary operation.

Commit 2f2007fbb did this partially, but there were two remaining
warts.  checkcondition_gin handled some uncertain cases by setting
the out-of-band recheck flag, some by returning TS_MAYBE, and some
by doing both.  Meanwhile, TS_execute arbitrarily converted a
TS_MAYBE result to TS_YES.  Thus, if checkcondition_gin chose to
only return TS_MAYBE, the outcome would be TS_YES with no recheck
flag, potentially resulting in wrong query outputs.

The case where this'd happen is if there were GIN_MAYBE entries
in the indexscan results passed to gin_tsquery_[tri]consistent,
which so far as I can see would only happen if the tidbitmap used
to accumulate indexscan results grew large enough to become lossy.

I initially thought of fixing this by ensuring we always set the
recheck flag as well as returning TS_MAYBE in uncertain cases.
But that errs in the other direction, potentially forcing rechecks
of rows that provably match the query (since the recheck flag
remains set even if TS_execute later finds that the answer must be
TS_YES).  Instead, let's get rid of the out-of-band recheck flag
altogether and rely on returning TS_MAYBE.  This requires exporting
a version of TS_execute that will actually return the full ternary
result of the evaluation ... but we likely should have done that
to start with.

Unfortunately it doesn't seem practical to add a regression test case
that covers this: the amount of data needed to cause the GIN bitmap to
become lossy results in a longer runtime than I think we want to have
in the tests.  (I'm wondering about allowing smaller work_mem settings
to ameliorate that, but it'd be a matter for a separate patch.)

Per bug #16865 from Dimitri Nüscheler.  Back-patch to v13 where
the faulty commit came in.

Discussion: https://postgr.es/m/16865-4ffdc3e682e6d75b@postgresql.org

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