Replace gnu coreutils with pure bash

[Costinteo]

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.