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!

defs.h

@@ -11,6 +11,13 @@
 #ifndef PUTTY_DEFS_H
 #define PUTTY_DEFS_H
 
+#ifndef FALSE
+#define FALSE 0
+#endif
+#ifndef TRUE
+#define TRUE 1
+#endif
+
 typedef struct conf_tag Conf;
 typedef struct backend_tag Backend;
 typedef struct terminal_tag Terminal;