postgres_fdw: Don't push down certain full joins.

If there's a filter condition on either side of a full outer join,
it is neither correct to attach it to the join's ON clause nor to
throw it into the toplevel WHERE clause.  Just don't push down the
join in that case.

To maximize the number of cases where we can still push down full
joins, push inner join conditions into the ON clause at the first
opportunity rather than postponing them to the top-level WHERE
clause.  This produces nicer SQL, anyway.

This bug was introduced in e4106b25.

Ashutosh Bapat, per report from Rajkumar Raghuwanshi.