Added separate queue for "system" APCs that get called even when the
thread is not in an alertable state.
Specify the select_request timeout as absolute value so that we can
restart the request when interrupted.

diff --git a/scheduler/thread.c b/scheduler/thread.c
index cea1156..3735a11 100644
--- a/scheduler/thread.c
+++ b/scheduler/thread.c
@@ -638,6 +638,7 @@
     {
         struct queue_apc_request *req = server_alloc_req( sizeof(*req), 0 );
         req->handle = hthread;
+        req->user   = 1;
         req->func   = func;
         req->param  = (void *)data;
         ret = !server_call( REQ_QUEUE_APC );