pgsql: Cleanup the usage of ScanDirection: use the symbolic names for

Log Message:
-----------
Cleanup the usage of ScanDirection: use the symbolic names for the
possible ScanDirection alternatives rather than magic numbers
(-1, 0, 1). Also, use the ScanDirection macros in a few places
rather than directly checking whether `dir == ForwardScanDirection'
and the like. Per patch from James William Pye. His patch also
changed ScanDirection to be a "char" rather than an enum, which
I haven't applied.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.206 -> r1.207)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.206&r2=1.207)
pgsql/src/backend/executor:
execMain.c (r1.266 -> r1.267)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c.diff?r1=1.266&r2=1.267)
pgsql/src/backend/tcop:
pquery.c (r1.98 -> r1.99)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/pquery.c.diff?r1=1.98&r2=1.99)