Use SIGUSR1 instead of SIGSTOP to suspend threads.

diff --git a/scheduler/client.c b/scheduler/client.c
index ef6db59..466fef7 100644
--- a/scheduler/client.c
+++ b/scheduler/client.c
@@ -653,6 +653,8 @@
     sigaddset( &block_set, SIGIO );
     sigaddset( &block_set, SIGINT );
     sigaddset( &block_set, SIGHUP );
+    sigaddset( &block_set, SIGUSR1 );
+    sigaddset( &block_set, SIGUSR2 );
 
     /* receive the first thread request fd on the main socket */
     NtCurrentTeb()->request_fd = receive_fd( &dummy_handle );