Use stdbool more widely

We already use the C99 bool type from stdbool.h in a few places.  However
there are many other places we represent boolean values as plain ints.
This patch changes that.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -70,7 +70,7 @@ char *join_path(const char *path, const char *name)
 	return str;
 }
 
-int util_is_printable_string(const void *data, int len)
+bool util_is_printable_string(const void *data, int len)
 {
 	const char *s = data;
 	const char *ss, *se;