Replace gnu coreutils with pure bash

Ideally the only requirements for shellsec would be ``bash`` and ``openssl``.

Bash parameter expansion is probably powerful enough to replace ``grep``. But it has to be replaced with iterations through the password file opened as a buffer, which kinda sucks (I think).

I also use ``head`` to get a certain length from ``/dev/urandom``, unsure what could replace it.

I don't know about ``tr``, though. Seems like the best way to get output from ``/dev/urandom``, but I haven't researched much.

There's also a single ``sed`` call in the script, that deletes a password. Again, this could be replaced, but it probably has to load the whole password file in memory, which sucks.