recheck cancellation disabled flag after syscall returns EINTR

we already checked before making the syscall, but it's possible that a
signal handler interrupted the blocking syscall and disabled
cancellation, and that this is the cause of EINTR. in this case, the
old behavior was testably wrong.

src/thread/cancel_impl.c

@@ -27,7 +27,7 @@ long (__syscall_cp)(long nr, long u, long v, long w, long x, long y, long z)
 	r = __syscall_cp_asm(&self->cp_sp, nr, u, v, w, x, y, z);
 	self->cp_ip = old_ip;
 	self->cp_sp = old_sp;
-	if (r == -EINTR && self->cancel) __cancel();
+	if (r == -EINTR && self->cancel && !self->canceldisable) __cancel();
 	return r;
 }