In preparation of adding OpenBSD support:
OpenBSD doesn't support timex but adjusting frequency using the
adjfreq(2) system call. Add a privops method PRV_AdjustFreq() to allow
unprivileged processes to set the time.
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.
Following the removal of gettimeofday() calls, change PRV_SetTime() to
use clock_settime() instead of settimeofday(). Only CLOCK_REALTIME is
supported for now.
Move the res_init() call from do_name_to_ipaddress() into a separate
privops operation. Use it in ntp_sources and avoid unnecessary
res_init() calls in the main thread.
Rename PRV_Initialise() to PRV_StartHelper() and add a new
initialisation function, which just sets the helper fd to -1. Move
the initialision/finalisation calls from the system drivers to main.c.
If privops is not included in the build, define empty macros for the
function names, so their calls don't have to be wrapped in #ifdefs.
