This is just mods to the tmcd "ifconfig" command to include an MTU= arg.
Right now we don't have anything in the DB for MTU, so tmcd is just returning
"MTU=" which says to not explicitly set the MTU.

It also includes the basic client-side support which I have tested on a
physical interface with MTU=1500. Further changes will be needed to DTRT
on virtual interfaces and their physical carrier interface.

But the hope is to get the client-side part nailed down before the next
set of images are rolled, so that we will be ready when support for the
front-side (UI and DB state) get added.