Now use a sitevar, general/allowjumboframes, rather than MAINSITE
to determine whether we should even attempt any jumbo frames magic.

Use a per-link/lan setting rather than the hacky per-experiment
setting to let the user decide if they want to use jumbos. In NS
world, we already had a link/lan method (set-settings) to specify
virt_lan_settings which is where it winds up now.

Client-side fixes to make jumbos work with vnodes.