correctly handle old kernels without FUTEX_WAIT_BITSET

futex returns EINVAL, not ENOSYS, when op is not supported.
unfortunately this looks just like EINVAL from other causes, and we
end up running the fallback code and getting EINVAL again. fortunately
this case should be rare since correct code should not generate EINVAL
anyway.

src/thread/__timedwait.c

@@ -16,7 +16,7 @@ static int do_wait(volatile int *addr, int val, clockid_t clk, const struct time
 		if (clk == CLOCK_REALTIME) flag = FUTEX_CLOCK_REALTIME;
 		if (cp) r = -__syscall_cp(SYS_futex, addr, FUTEX_WAIT_BITSET|flag, val, at, 0, -1);
 		else r = -__syscall(SYS_futex, addr, FUTEX_WAIT_BITSET|flag, val, at, 0, -1);
-		if (r != ENOSYS) goto done;
+		if (r != EINVAL) goto done;
 	}
 	if (clock_gettime(clk, &to)) return EINVAL;
 	to.tv_sec = at->tv_sec - to.tv_sec;