Mark a few functions as __attribute__((noreturn)).

This is mostly to make static analysers and compiler warnings a bit
happier - now they know that a call to, say, modalfatalbox() means
they don't have to worry about what the rest of the function will do.

defs.h

@@ -113,4 +113,10 @@ typedef struct PacketProtocolLayer PacketProtocolLayer;
     TYPECHECK(object == &((type *)0)->field,                            \
               ((type *)(((char *)(object)) - offsetof(type, field))))
 
+#if defined __GNUC__ || defined __clang__
+#define NORETURN __attribute__((__noreturn__))
+#else
+#define NORETURN
+#endif
+
 #endif /* PUTTY_DEFS_H */