pgsql: Instead of using a numberOfRequiredKeys count to distinguish

Log Message:
-----------
Instead of using a numberOfRequiredKeys count to distinguish required
and non-required keys in a btree index scan, mark the required scankeys
with private flag bits SK_BT_REQFWD and/or SK_BT_REQBKWD. This seems
at least marginally clearer to me, and it eliminates a wired-into-the-
data-structure assumption that required keys are consecutive. Even though
that assumption will remain true for the foreseeable future, having it
in there makes the code seem more complex than necessary.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtsearch.c (r1.100 -> r1.101)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtsearch.c.diff?r1=1.100&r2=1.101)
nbtutils.c (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.68&r2=1.69)
pgsql/src/include/access:
nbtree.h (r1.89 -> r1.90)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/nbtree.h.diff?r1=1.89&r2=1.90)