I've been using lo0 to debug code intended to run on large-MTU
interfaces.  Turns out there's a gotcha with BPF and loopback MTU: we
don't allow BPF capture buffer larger than 32k by default, but the
default loopback MTU is (32K + MHLEN + MLEN).

To date I've kluged around that with a couple of lines which add
SIOCSIFMTU support to if_loop.c.  I end up needing that every few
months for various tests. It's a trivial increase in size.  Does
anyone object if I commit the changes, and defopt an option to
increase the max MTU?

Longer-term, it'd be nice to make sure the BPF capture buffers can
hold an entire frame on all configured interfaces. Do we support any
interfaces beside HIPPI and loopback which use MTUs bigger than 32k?