Add option to set the DHCP Interface MTU parameter

[craigpratt]

In order to support tunneling inter-AP traffic over vxlans on a network with 1500-byte MTUs the MTU on the micronets network needs to be reduced so the addition of the vxlan headers doesn't exceed the 1500-byte limit.

The DHCP "Interface MTU Option" is supposed to accomplish this proactively - and avoid having to resort to Path MTU Discovery - which can cause some latency during connection establishment. The description of this option is below.

It probably makes sense to add a MTU size parameter to the Micronets definition. But it's TBD if this is set as a NetReach AP parameter or as an agent configuration value.

   This option specifies the MTU to use on this interface.  The MTU is
   specified as a 16-bit unsigned integer.  The minimum legal value for
   the MTU is 68.

   The code for this option is 26, and its length is 2.

    Code   Len      MTU
   +-----+-----+-----+-----+
   |  26 |  2  |  m1 |  m2 |
   +-----+-----+-----+-----+```