Add compile time sanity check for interface name lengths.

Because our interfaces include UNIX domain sockets, we are using
UNIX_PATH_MAX of 108 bytes for our interface names.  Even though
IF_NAMESIZE is much smaller (16 bytes), there is nothing that
guarantees this.  This patch adds a compile time sanity check that
tests whether the interface name will fit.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>

config.h

@@ -32,6 +32,10 @@
 
 #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;