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>
diff --git a/util.c b/util.c
index e87d6c1..2347af9 100644
--- a/util.c
+++ b/util.c
@@ -70,7 +70,7 @@
 	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;