Added decl_early_init() which is called when PLAYAGAIN is supported.

decl_early_init() is called when we are starting a game.  On first
start, it validates that global state is in the expected state.
When called on subsequent starts, it initializes global state to
expected state.

include/extern.h

@@ -251,6 +251,9 @@ E void FDECL(destroy_drawbridge, (int, int));
 /* ### decl.c ### */
 
 E void NDECL(decl_init);
+#ifdef PLAYAGAIN
+E void NDECL(decl_early_init);
+#endif
 
 /* ### detect.c ### */