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.

sys_linux.c

@@ -658,6 +658,9 @@ SYS_Linux_EnableSystemCallFilter(int level, SYS_ProcessContext context)
        modules are installed and enabled on the system). */
     if (default_action != SCMP_ACT_ALLOW)
       PRV_StartHelper();
+  } else if (context == SYS_PRIVOPS_HELPER) {
+    /* The privops helper on Linux doesn't have any filter loaded */
+    return;
   }
 
   ctx = seccomp_init(default_action);