ntp: add support for software timestamping on Linux

Enable SCM_TIMESTAMPING control messages and the socket's error queue in
order to receive our transmitted packets with a more accurate transmit
timestamp. Add a new file for Linux-specific NTP I/O and implement
processing of these messages there.

sys_linux.c

@@ -501,6 +501,9 @@ SYS_Linux_EnableSystemCallFilter(int level)
 #endif
     { SOL_SOCKET, SO_BROADCAST }, { SOL_SOCKET, SO_REUSEADDR },
     { SOL_SOCKET, SO_TIMESTAMP }, { SOL_SOCKET, SO_TIMESTAMPNS },
+#ifdef HAVE_LINUX_TIMESTAMPING
+    { SOL_SOCKET, SO_SELECT_ERR_QUEUE }, { SOL_SOCKET, SO_TIMESTAMPING },
+#endif
   };
 
   const static int fcntls[] = { F_GETFD, F_SETFD };