Broke rc.storage into two phases, local blockstores and remote blockstores.
Setup of the former will also pick a best candidate for an old-school
"extrafs" and put the info in /var/emulab/boot/extrafs. This will be a
single line with one of DISK=foo, PART=foo, or FS=foo depending on whether
it found an available full disk, disk partition, or mounted filesystem
that we can use for mkextrafs (in the first two cases) or where we can
mooch off of (the last). This is only used in os_mountextrafs() right now;
i.e., I have NOT changed the mkextrafs script. So explicit invocations
by the user could still screw things up.

I have tested this with local blockstores and a non-nfs experiment
on both Linux and FreeBSD to make sure the most common sharing of space
works. I have not made any new images and I have not yet tested to make
sure I did not break non-blockstore, non-nfs experiments (i.e., where
we really should run mkextrafs).

So maybe don't make any new images til I get back, or else be prepared
to clean up after me.