pgsql: Fix another oversight in CustomScan patch.

Fix another oversight in CustomScan patch.

execCurrent.c's search_plan_tree() must recognize a CustomScan on the
target relation. This would only be helpful for custom providers that
support CurrentOfExpr quals, which is probably a bit far-fetched, but
it's not impossible I think. But even without assuming that, we need
to recognize a scanned-relation match so that we will properly throw
error if the desired relation is being scanned with both a CustomScan
and a regular scan (ie, self-join).

Also recognize ForeignScanState for similar reasons. Supporting WHERE
CURRENT OF on a foreign table is probably even more far-fetched than
it is for custom scans, but I think in principle you could do it with
postgres_fdw (or another FDW that supports the ctid column). This
would be a back-patchable bug fix if existing FDWs handled CurrentOfExpr,
but I doubt any do so I won't bother back-patching.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/081a6048cff07a83591ebcb08b676a771ae58d2b

Modified Files
--------------
src/backend/executor/execCurrent.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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