Better variable quoting in passwd-gen.sh.

Better variable quoting in passwd-gen.sh.

With the previous version, the unquoted `SET` passed to `tr` could
cause problems if Bash exapanded it to a filename.  Now everything is
nicely quoted, using braces and everything.  I also use
`${paramter:-word}` for all the configuration variables, which makes
it easy to override them without editing the script.  For example, you
can now run:

  $ DEVICE=/dev/urandom SET="[:alnum:][:punct:]" passwd-gen.sh 10