privops: enable system call filter

In preparation of OpenBSD support, add SYS_EnableSystemCallFilter() call
to PRV_StartHelper().

In OpenBSD the privops helper will use a system call filter (pledge(2)),
whereas in Linux the privops helper doesn't use any system call filter
at the moment.

Modify Unit test ntp_sources call to PRV_Initialise() with parameter
scfilter_level set to 0.

main.c

@@ -650,7 +650,7 @@ int main
   /* Write our pidfile to prevent other instances from running */
   write_pidfile();
 
-  PRV_Initialise();
+  PRV_Initialise(scfilter_level);
   LCL_Initialise();
   SCH_Initialise();
   SCK_Initialise(address_family);