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!

ssh.c

@@ -25,13 +25,6 @@
 #define GSS_CTXT_MAYFAIL (1<<3)	/* Context may expire during handshake */
 #endif
 
-#ifndef FALSE
-#define FALSE 0
-#endif
-#ifndef TRUE
-#define TRUE 1
-#endif
-
 /*
  * Packet type contexts, so that ssh2_pkt_type can correctly decode
  * the ambiguous type numbers back into the correct type strings.