unify the use of FUTEX_PRIVATE

The flag 1<<7 is used in several places for different purposes that are
not always easy to distinguish. Mark those usages that correspond to the
flag that is used by the kernel for futexes.
diff --git a/src/thread/__timedwait.c b/src/thread/__timedwait.c
index 13d8465..d2079c8 100644
--- a/src/thread/__timedwait.c
+++ b/src/thread/__timedwait.c
@@ -14,7 +14,7 @@
 	int r;
 	struct timespec to, *top=0;
 
-	if (priv) priv = 128;
+	if (priv) priv = FUTEX_PRIVATE;
 
 	if (at) {
 		if (at->tv_nsec >= 1000000000UL) return EINVAL;