fix invalid pointer in synccall (multithread setuid, etc.)

the head pointer was not being reset between calls to synccall, so any
use of this interface more than once would build the linked list
incorrectly, keeping the (now invalid) list nodes from the previous
call.

src/thread/synccall.c

@@ -58,6 +58,7 @@ void __synccall(void (*func)(void *), void *ctx)
 	sem_init(&chaindone, 0, 0);
 	sem_init(&chainlock, 0, 1);
 	chainlen = 0;
+	head = 0;
 	callback = func;
 	context = ctx;