No need to just go ahead and source files in potentially
unavailable directories, really.

"df -l" is going to hurt (or at least spam) if it's a diskless node;
"df -P" helps maintain output parseability.  So be it.

As was found out by Vladimir Karpinsky (thanks for patience!),
the autochosen directory might still have too restrictive mount
options -- nodev and/or noexec.  Hopefully the diags are a bit
better and faster by now.

It happens that if the host environment isn't particularly
tuned up for package builds already then bin/mktmpdir might
come up with a directory outside hasher-allowed prefix list;
now that's a shame and not a Christmas gift, clearly.

Thanks Vladimir Karpinsky for pointing this problem out too.

The fallback case of building in a brother directory moved
from the last line of code to the first one becoming more
explicit along the way.

Support for slash-containing argument (being a tmpdir name
template prefix) has been added.

With not-that-recent mkimage-profiles development,
it's no longer apparent that at least a gigabyte
of free space is required to build something useful
(at least for the tests, like syslinux.iso).

In short, the guesser cutoff margin is now 256M.

Just in case the build.log will be inobvious, and it's easy to diagnose
automatically.  Thanks Andrey Stroganov for this use case.

Thanks for improving the initial implementation go to raorn@ for kind
commit lynch and to Yuri Bushmelev for actually suggesting something
more concise.

BTW the "1024" magic number was taken out of thin air:
the "no free space" errors are most likely to happen while
forming/populating a chroot (apt/rpm errors out) and chroots are
roughly two orders of magnitude heftier than a megabyte.

- s/stage2/install2/g
- added pkg/lists
- initial README (ru_RU.KOI8-R)
- mkimage topdir is in image.in/, really (off-tree build)
  + initial bin/mktmpdir
  + BUILDDIR (works)
- s/cd/iso/
- drop --with-distro (considered harmful)