files.mips: the kbd driver is used by vt(4) too

Sponsored by:	DARPA, AFRL
Provide a means for loaders to control which file system to use. This
to counteract the default behaviour of always trying each and every
file system until one succeeds, or the open fails. The problem with the
loader is that we've implemented features based on this behavior. The
handling of compressed files is a good example of this. However, it is
in general highly undesirable to not have a one-time probe (or taste
in the geom lingo), followed by something similar to a mount whenever
we (first) read from a device. Everytime we go to the same device, we
can reasonably assume it (still) has the same file system. For file
systems that need to do far more that a trivial read of a super block,
not having something similar to a mount operation is disastrous from
a performance (and thus usability) perspective.

But, again, since we've implemented features based on this stateless
approach, things can get complicated quickly if and when we want to
change this. And yet, we sometimes do need stateful behaviour.

For this reason, this change simply introduces exclusive_file_system.
When set to the fsops of the file system to use, the open call will
only try this file system. Setting it to NULL restores the default
behaviour. It's a low-cost (low-brow?) approach to provide enough
control without re-implementing the guts of the loader.

A good example of when this is useful is when we're trying to load
files out of a container (say, a software packaga) that itself lives
on a file system or is fetched over the network. While opening the
container can be done in the normal stateless manner, once it is
opened, subsequent opens should only consider the container.

Obtained from:	Juniper Networks, Inc.