Adopt C99 <stdbool.h>'s true/false.

This commit includes <stdbool.h> from defs.h and deletes my
traditional definitions of TRUE and FALSE, but other than that, it's a
100% mechanical search-and-replace transforming all uses of TRUE and
FALSE into the C99-standardised lowercase spellings.

No actual types are changed in this commit; that will come next. This
is just getting the noise out of the way, so that subsequent commits
can have a higher proportion of signal.

sshbpp.h

@@ -99,8 +99,8 @@ struct DataTransferStats {
 #define DTS_CONSUME(stats, direction, size)             \
     ((stats)->direction.running &&                      \
      (stats)->direction.remaining <= (size) ?           \
-     ((stats)->direction.running = FALSE, TRUE) :       \
-     ((stats)->direction.remaining -= (size), FALSE))
+     ((stats)->direction.running = false, true) :       \
+     ((stats)->direction.remaining -= (size), false))
 
 BinaryPacketProtocol *ssh2_bpp_new(
     LogContext *logctx, struct DataTransferStats *stats, int is_server);