Allocate 4 bytes of unused header space for an "Application ID". Add

the "PRAGMA application_id" command to set and query this identifier.
Add the "magic.txt" file to show how the posix file command might use
this application id.

FossilOrigin-Name: 28c9e7fdee2471a3026ee05ff591194d5f398131

src/btree.h

@@ -140,6 +140,7 @@ int sqlite3BtreeNewDb(Btree *p);
 #define BTREE_TEXT_ENCODING       5
 #define BTREE_USER_VERSION        6
 #define BTREE_INCR_VACUUM         7
+#define BTREE_APPLICATION_ID      8
 
 /*
 ** Values that may be OR'd together to form the second argument of an