[PATCH] Support SK_SEARCHNULL / SK_SEARCHNOTNULL for heap-only scans

Hi hackers,

A colleague of mine wanted to use a ScanKey with SK_SEARCHNULL flag
for a heap-only scan (besides other ScanKeys) and discovered that the
result differs from what he would expect. Turned out that this is
currently not supported as it is explicitly stated in skey.h.

Although several workarounds come to mind this limitation may be
really of inconvenience for the extension authors, and implementing
corresponding support seems to be pretty straightforward.

The attached patch does this.

--
Best regards,
Aleksander Alekseev