sys_netbsd: allow running without root privileges

On NetBSD programs with write access to /dev/clockctl can adjust or set
the system clock without the root privileges. Add a function to drop the
privileges and check if the process has write access to the device to
get a more descriptive error message when the chrony uid/gid doesn't
match the owner of the device.

sys.c

@@ -113,6 +113,8 @@ void SYS_DropRoot(uid_t uid, gid_t gid)
 {
 #if defined(LINUX) && defined (FEAT_PRIVDROP)
   SYS_Linux_DropRoot(uid, gid);
+#elif defined(NETBSD) && defined(FEAT_PRIVDROP)
+  SYS_NetBSD_DropRoot(uid, gid);
 #else
   LOG_FATAL(LOGF_Sys, "dropping root privileges not supported");
 #endif