pgsql: Fix some oversights in BRIN patch.

Fix some oversights in BRIN patch.

Remove HeapScanDescData.rs_initblock, which wasn't being used for anything
in the final version of the patch.

Fix IndexBuildHeapScan so that it supports syncscan again; the patch
broke synchronous scanning for index builds by forcing rs_startblk
to zero even when the caller did not care about that and had asked
for syncscan.

Add some commentary and usage defenses to heap_setscanlimits().

Fix heapam so that asking for rs_numblocks == 0 does what you would
reasonably expect.  As coded it amounted to requesting a whole-table
scan, because those "--x <= 0" tests on an unsigned variable would
behave surprisingly.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/35ac618a7c4602b792160ae0d77b6dfb289f517e

Modified Files
--------------
src/backend/access/heap/heapam.c |   30 ++++++++++++++++++++----------
src/backend/catalog/index.c      |   12 ++++++++++--
src/include/access/relscan.h     |    4 ++--
3 files changed, 32 insertions(+), 14 deletions(-)