ntp: add support for HW timestamping on Linux

Add a new directive to specify interfaces which should be used for HW
timestamping. Extend the Linux ntp_io initialization to enable HW
timestamping, configure the RX filter using the SIOCSHWTSTAMP ioctl,
open their PHC devices, and track them as hwclock instances. When
messages with HW timestamps are received, use the PTP_SYS_OFFSET ioctl
to make PHC samples for hwclock.

ntp_io.c

@@ -363,6 +363,9 @@ NIO_Initialise(int family)
 
 #ifdef HAVE_LINUX_TIMESTAMPING
   NIO_Linux_Initialise();
+#else
+  if (ARR_GetSize(CNF_GetHwTsInterfaces()))
+    LOG_FATAL(LOGF_NtpIO, "HW timestamping not supported");
 #endif
 
   recv_messages = ARR_CreateInstance(sizeof (struct Message));