Update text in 4.3.3 to include first-hop MTU in the tests:

   o  message size: a message whose size (including the GIST header,
      GIST objects and any NSLP payload, and an allowance for the IP and
      transport layer encapsulation required by D-mode) exceeds a
      fragmentation-related threshold MUST be sent over C-mode, using a
      messaging association that supports fragmentation and reassembly
      internally.  The allowance for IP and transport layer
      encapsulation is 64 bytes.  The message size MUST NOT exceed the
      least of the following three quantities: the Path MTU to the next
      peer (if known), the first-hop MTU, and 576 bytes.  The same limit
      applies to IPv4 and IPv6.