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.

defs.h

@@ -13,6 +13,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <stdbool.h>
 
 #if defined _MSC_VER && _MSC_VER < 1800
 /* Work around lack of inttypes.h in older MSVC */
@@ -22,13 +23,6 @@
 #include <inttypes.h>
 #endif
 
-#ifndef FALSE
-#define FALSE 0
-#endif
-#ifndef TRUE
-#define TRUE 1
-#endif
-
 typedef struct conf_tag Conf;
 typedef struct terminal_tag Terminal;