sys_macosx: drop root privileges

Run chronyd as a non-privileged user, using the privops helper to
perform adjtime(), settimeofday() and bind() functions on its behalf.

sys.c

@@ -92,6 +92,8 @@ void SYS_DropRoot(uid_t uid, gid_t gid)
   SYS_Linux_DropRoot(uid, gid);
 #elif defined(NETBSD) && defined(FEAT_PRIVDROP)
   SYS_NetBSD_DropRoot(uid, gid);
+#elif defined(MACOSX) && defined(FEAT_PRIVDROP)
+  SYS_MacOSX_DropRoot(uid, gid);
 #else
   LOG_FATAL(LOGF_Sys, "dropping root privileges not supported");
 #endif