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_macosx.h

@@ -31,6 +31,7 @@
 #define GOT_SYS_MACOSX_H
 
 void SYS_MacOSX_SetScheduler(int SchedPriority);
+void SYS_MacOSX_DropRoot(uid_t uid, gid_t gid);
 void SYS_MacOSX_Initialise(void);
 void SYS_MacOSX_Finalise(void);