Only chsh to zsh if zsh is not the shell

This PR checks the user's current shell before making the chsh call.  This is a good thing because the chsh call requires a password input, so it'd be nice to avoid that where possible.
$SHELL is set, but I'm not sure how reliable it is.  I tried using it originally, but it reported as bash even when called from the command-line in zsh.  I was probably doing something wrong, though, as it's working as expected now.

That said, I think $SHELL is far preferable to using the parent process, because we want to check the user's default shell, not what shell they happened to be using when they ran the script.  If for some weird reason they're a bash-defaulted user in zsh, chsh wouldn't be called.