Add os_process_data and related

This is meant to be the same for per-OS configuration as arch_process_data
is for per-arch configuration.
diff --git a/proc.h b/proc.h
index b1b22b5..8e1eec3 100644
--- a/proc.h
+++ b/proc.h
@@ -147,6 +147,7 @@
 	   that case this process is waiting to be collected.  */
 	Process * leader;
 
+	struct os_process_data os;
 	struct arch_process_data arch;
 };