Move strategy numbers to include/access/stratnum.h

For upcoming BRIN opclasses, it's convenient to have strategy numbers
defined in a single place.  Since there's nothing appropriate, create
it.  The StrategyNumber typedef now lives there, as well as existing
strategy numbers for B-trees (from skey.h) and R-tree-and-friends (from
gist.h).  skey.h is forced to include stratnum.h because of the
StrategyNumber typedef, but gist.h is not; extensions that currently
rely on gist.h for rtree strategy numbers might need to add a new

A few .c files can stop including skey.h and/or gist.h, which is a nice
side benefit.

Per discussion:
https://www.postgresql.org/message-id/20150514232132.GZ2523@alvh.no-ip.org

Authored by Emre Hasegeli and Álvaro.

(It's not clear to me why bootscanner.l has any #include lines at all.)

src/include/access/skey.h

@@ -15,31 +15,10 @@
 #define SKEY_H
 
 #include "access/attnum.h"
+#include "access/stratnum.h"
 #include "fmgr.h"
 
 
-/*
- * Strategy numbers identify the semantics that particular operators have
- * with respect to particular operator classes.  In some cases a strategy
- * subtype (an OID) is used as further information.
- */
-typedef uint16 StrategyNumber;
-
-#define InvalidStrategy ((StrategyNumber) 0)
-
-/*
- * We define the strategy numbers for B-tree indexes here, to avoid having
- * to import access/nbtree.h into a lot of places that shouldn't need it.
- */
-#define BTLessStrategyNumber			1
-#define BTLessEqualStrategyNumber		2
-#define BTEqualStrategyNumber			3
-#define BTGreaterEqualStrategyNumber	4
-#define BTGreaterStrategyNumber			5
-
-#define BTMaxStrategyNumber				5
-
-
 /*
  * A ScanKey represents the application of a comparison operator between
  * a table or index column and a constant.  When it's part of an array of