pgsql: Tweak hash index AM to use the new ReadOrZeroBuffer bufmgr API

Log Message:
-----------
Tweak hash index AM to use the new ReadOrZeroBuffer bufmgr API when fetching
pages it intends to zero immediately. Just to show there is some use for that
function besides WAL recovery :-).
Along the way, fold _hash_checkpage and _hash_pageinit calls into _hash_getbuf
and friends, instead of expecting callers to do that separately.

Modified Files:
--------------
pgsql/contrib/pgstattuple:
pgstattuple.c (r1.26 -> r1.27)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgstattuple/pgstattuple.c.diff?r1=1.26&r2=1.27)
pgsql/src/backend/access/hash:
hash.c (r1.93 -> r1.94)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c.diff?r1=1.93&r2=1.94)
hashinsert.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashinsert.c.diff?r1=1.44&r2=1.45)
hashovfl.c (r1.56 -> r1.57)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashovfl.c.diff?r1=1.56&r2=1.57)
hashpage.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashpage.c.diff?r1=1.66&r2=1.67)
hashsearch.c (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashsearch.c.diff?r1=1.48&r2=1.49)
hashutil.c (r1.51 -> r1.52)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashutil.c.diff?r1=1.51&r2=1.52)
pgsql/src/include/access:
hash.h (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/hash.h.diff?r1=1.79&r2=1.80)