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.h

@@ -38,6 +38,7 @@ extern void SYS_Finalise(void);
 typedef enum {
   SYS_MAIN_PROCESS,
   SYS_NTSKE_HELPER,
+  SYS_PRIVOPS_HELPER,
 } SYS_ProcessContext;
 
 /* Switch to the specified user and group in given context */