Centralise TRUE and FALSE definitions into defs.h.

This removes a lot of pointless duplications of those constants.

Of course, _ideally_, I should upgrade to C99 bool throughout the code
base, replacing TRUE and FALSE with true and false and tagging
variables explicitly as bool when that's what they semantically are.
But that's a much bigger piece of work, and shouldn't block this
trivial cleanup!

telnet.c

@@ -8,13 +8,6 @@
 
 #include "putty.h"
 
-#ifndef FALSE
-#define FALSE 0
-#endif
-#ifndef TRUE
-#define TRUE 1
-#endif
-
 #define	IAC	255		       /* interpret as command: */
 #define	DONT	254		       /* you are not to use option */
 #define	DO	253		       /* please, you use option */