Move the network interface into its own header file.

Up until now, the users of the interface data structure simply access
its fields without restriction.  This patch takes the first step
towards abstracting this data structure by giving it a file of its
very own.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>

config.h

@@ -26,25 +26,12 @@
 #include "ds.h"
 #include "dm.h"
 #include "filter.h"
+#include "interface.h"
 #include "mtab.h"
 #include "transport.h"
 #include "servo.h"
 #include "sk.h"
 
-#define MAX_IFNAME_SIZE 108 /* = UNIX_PATH_MAX */
-
-#if (IF_NAMESIZE > MAX_IFNAME_SIZE)
-#error if_namesize larger than expected.
-#endif
-
-/** Defines a network interface, with PTP options. */
-struct interface {
-	STAILQ_ENTRY(interface) list;
-	char name[MAX_IFNAME_SIZE + 1];
-	char ts_label[MAX_IFNAME_SIZE + 1];
-	struct sk_ts_info ts_info;
-};
-
 struct config {
 	/* configured interfaces */
 	STAILQ_HEAD(interfaces_head, interface) interfaces;

interface.h

@@ -0,0 +1,28 @@
+/**
+ * @file interface.h
+ * @brief Implements network interface data structures.
+ * @note Copyright (C) 2020 Richard Cochran <richardcochran@gmail.com>
+ * @note SPDX-License-Identifier: GPL-2.0+
+ */
+#ifndef HAVE_INTERFACE_H
+#define HAVE_INTERFACE_H
+
+#include <sys/queue.h>
+#include "sk.h"
+
+#define MAX_IFNAME_SIZE 108 /* = UNIX_PATH_MAX */
+
+#if (IF_NAMESIZE > MAX_IFNAME_SIZE)
+#error if_namesize larger than expected.
+#endif
+
+/** Defines a network interface, with PTP options. */
+struct interface {
+	STAILQ_ENTRY(interface) list;
+	char name[MAX_IFNAME_SIZE + 1];
+	char ts_label[MAX_IFNAME_SIZE + 1];
+	struct sk_ts_info ts_info;
+};
+
+#endif
+