Make dhclient set interface MTU if it was provided.
This version implements MTU setting in dhclient itself before it runs dhclient-script.
Also, to be able to test this with devd, I've modified if_vtnet(4) to send LINK_(UP|DOWN) (inl. the case when MTU is being changed): https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=209427
So far it works well in my setup...