Clean up manipulations of hash indexes' hasho_flag field.

Standardize on testing a hash index page's type by doing
	(opaque->hasho_flag & LH_PAGE_TYPE) == LH_xxx_PAGE
Various places were taking shortcuts like
	opaque->hasho_flag & LH_BUCKET_PAGE
which while not actually wrong, is still bad practice because
it encourages use of
	opaque->hasho_flag & LH_UNUSED_PAGE
which *is* wrong (LH_UNUSED_PAGE == 0, so the above is constant false).
hash_xlog.c's hash_mask() contained such an incorrect test.

This also ensures that we mask out the additional flag bits that
hasho_flag has accreted since 9.6.  pgstattuple's pgstat_hash_page(),
for one, was failing to do that and was thus actively broken.

Also fix assorted comments that hadn't been updated to reflect the
extended usage of hasho_flag, and fix some macros that were testing
just "(hasho_flag & bit)" to use the less dangerous, project-approved
form "((hasho_flag & bit) != 0)".

Coverity found the bug in hash_mask(); I noted the one in
pgstat_hash_page() through code reading.

contrib/pgstattuple/pgstattuple.c

@@ -453,7 +453,7 @@ pgstat_hash_page(pgstattuple_type *stat, Relation rel, BlockNumber blkno,
 		HashPageOpaque opaque;
 
 		opaque = (HashPageOpaque) PageGetSpecialPointer(page);
-		switch (opaque->hasho_flag)
+		switch (opaque->hasho_flag & LH_PAGE_TYPE)
 		{
 			case LH_UNUSED_PAGE:
 				stat->free_space += BLCKSZ;