Tux3 command interpreter

Tux3 now has a command interpreter to aid in debugging, with commands
like:

   tux3 make foodev

that makes a filesystem on device foodev, which can also be a file.  It
normally is a file for me and I make it sparse like this:

   dd if=/dev/zero of=foodev bs=1 count=1 seek=100K

I can see how many blocks tux3 actually used in it by:

   du foodev

less the few blocks that the sparse file has for metatdata and the
little blob of data at the end that dd puts there.  (I don't know how
to make dd truncate without writing, I am not sure it is possible.)

Then:

   echo "hello world!" | tux3 write foodev foo

creates and writes to file foo in foodev.  A subsequent write will
fail with EEXIST, which is maybe not quite what we want.

Finally:

  tux3 read foodev foo

outputs "hello world!".