Fix 'warning: egrep is obsolescent; using grep -E'

Manual BUILDDIR specification isn't exactly obvious
from this script (aiming to derive it automatically),
so at least point those interested at QUICKSTART
they might have long forgotten already or, gasp,
not even started reading.

Either /etc/hasher-priv/system or /etc/hasher-priv/user.d/$USER
must contain at least "allowed_mountpoints=/proc" for mkimage
to work for mkimage-profiles; thanks Daniil Golovanov for
providing feedback indicating the lack of the corresponding
checks.

The sort key field value has been off-by-one apparently,
"6" corresponds to "capacity" field of df -PT output
while the "available" one was meant:

Filesystem     Type  1024-blocks     Used Available Capacity Mounted on
/dev/sda9      ext4     15350768 12799972   1747980      88% /
tmpfs          tmpfs     1967204    27396   1939808       2% /tmp

1              2               3        4         5       6

This lapse has been five years old, looks like it's only
been masked by lower available space limit *and* filesystem
ordering by type so far.  It didn't hit me even now, rather
inspected the code while implementing volumes-profile-starterkit...

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)