Add physical MTU recommendation hint to network config via API.

include/ZeroTierOne.h

@@ -977,6 +977,11 @@ typedef struct
 	 */
 	unsigned int mtu;
 
+	/**
+	 * Recommended MTU to avoid fragmentation at the physical layer (hint)
+	 */
+	unsigned int physicalMtu;
+
 	/**
 	 * If nonzero, the network this port belongs to indicates DHCP availability
 	 *
@@ -1604,6 +1609,9 @@ typedef int (*ZT_PathCheckFunction)(
  * Note that this can take a few seconds the first time it's called, as it
  * will generate an identity.
  *
+ * TODO: should consolidate function pointers into versioned structure for
+ * better API stability.
+ *
  * @param node Result: pointer is set to new node instance on success
  * @param uptr User pointer to pass to functions/callbacks
  * @param now Current clock in milliseconds