[init] Remove concept of "shutdown exit flags"

Remove the concept of shutdown exit flags, and replace it with a
counter used to keep track of exposed interfaces that require devices
to remain active.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/main.c

@@ -85,7 +85,7 @@ __asmcall int main ( void ) {
 			shell();
 	}
 
-	shutdown ( SHUTDOWN_EXIT | shutdown_exit_flags );
+	shutdown_exit();
 
 	return 0;
 }