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.