Exclude pg_internal.init from BASE_BACKUP

Add docs to explain this for other backup mechanisms

Author: David Steele <david@pgmasters.net>
Reviewed-by: Petr Jelinek <petr.jelinek@2ndQuadrant.com> et al

src/include/utils/relcache.h

@@ -18,6 +18,11 @@
 #include "nodes/bitmapset.h"
 
 
+/*
+ * Name of relcache init file(s), used to speed up backend startup
+ */
+#define RELCACHE_INIT_FILENAME	"pg_internal.init"
+
 typedef struct RelationData *Relation;
 
 /* ----------------