sys_netbsd: use timex driver

Remove the driver functions based on adjtime() and switch to the new
timex driver, which is based on ntp_adjtime(). This allows chronyd to
control the kernel frequency, adjust the offset with sub-microsecond
accuracy, and set the kernel leap and sync status. A drawback is that
the maximum slew rate is now limited by the 500 ppm maximum frequency
offset, while adjtime() on NetBSD slewed by up to 5000 ppm.

configure

@@ -419,8 +419,7 @@ case $SYSTEM in
         echo "Configuring for $SYSTEM (using SunOS driver)"
     ;;
     NetBSD-* )
-        EXTRA_OBJECTS="sys_netbsd.o"
-        EXTRA_LIBS="-lkvm"
+        EXTRA_OBJECTS="sys_generic.o sys_netbsd.o sys_timex.o"
         try_clockctl=1
         add_def NETBSD
         echo "Configuring for $SYSTEM"