[gPXE] PATCH: Allow "dhcp" command to iterate over multiple interfaces like autoboot()

[Lars Kellogg-Stedman]

> Can you explain why -c is necessary to stay compatible with the
> current command?  The current command takes one argument and fails if
> the interface doesn't exist.

I wanted to preserve the "fail on unknown interface" semantics, even
with multiple interfaces on the command line.  I'm not sure in
retrospect if this matters or not.  It seemed worthwhile to have to
have both behaviors (because maybe the unknown interface is due to a
spelling error and it *should* fail?).  I'm not wedded to this option,
and if you think that removing it simplifies things I'm all for it.