Refactor reloption handling for index AMs in-core

This reworks the reloption parsing and build of a couple of index AMs by
creating new structures for each index AM's options.  This split was
already done for BRIN, GIN and GiST (which actually has a fillfactor
parameter), but not for hash, B-tree and SPGiST which relied on
StdRdOptions due to an overlap with the default option set.

This saves a couple of bytes for rd_options in each relcache entry with
indexes making use of relation options, and brings more consistency
between all index AMs.  While on it, add a couple of AssertMacro() calls
to make sure that utility macros to grab values of reloptions are used
with the expected index AM.

Author: Nikolay Shaplov
Reviewed-by: Amit Langote, Michael Paquier, Álvaro Herrera, Dent John
Discussion: https://postgr.es/m/4127670.gFlpRb6XCm@x200m

src/include/access/nbtree.h

@@ -18,6 +18,7 @@
 #include "access/itup.h"
 #include "access/sdir.h"
 #include "access/xlogreader.h"
+#include "catalog/pg_am_d.h"
 #include "catalog/pg_index.h"
 #include "lib/stringinfo.h"
 #include "storage/bufmgr.h"
@@ -680,6 +681,23 @@ typedef BTScanOpaqueData *BTScanOpaque;
 #define SK_BT_DESC			(INDOPTION_DESC << SK_BT_INDOPTION_SHIFT)
 #define SK_BT_NULLS_FIRST	(INDOPTION_NULLS_FIRST << SK_BT_INDOPTION_SHIFT)
 
+typedef struct BTOptions
+{
+	int32		varlena_header_;	/* varlena header (do not touch directly!) */
+	int			fillfactor;		/* page fill factor in percent (0..100) */
+	/* fraction of newly inserted tuples prior to trigger index cleanup */
+	float8		vacuum_cleanup_index_scale_factor;
+} BTOptions;
+
+#define BTGetFillFactor(relation) \
+	(AssertMacro(relation->rd_rel->relkind == RELKIND_INDEX && \
+				 relation->rd_rel->relam == BTREE_AM_OID), \
+	 (relation)->rd_options ? \
+	 ((BTOptions *) (relation)->rd_options)->fillfactor : \
+	 BTREE_DEFAULT_FILLFACTOR)
+#define BTGetTargetPageFreeSpace(relation) \
+	(BLCKSZ * (100 - BTGetFillFactor(relation)) / 100)
+
 /*
  * Constant definition for progress reporting.  Phase numbers must match
  * btbuildphasename.