`-i` and `-s` flag are not supported properly by the CLI

[squell]

sudo -i and sudo -s should also run without arguments:

$ sudo-rs -i
sudo-rs: `': command not found

In original sudo, this gives you a shell into the root home directory. Similarly sudo -s is of course what everybody should use instead of sudo bash or sudo su.

Note that this should not be dealt with in the sudo-exec crate, since that runs post-permission checks (we must check that the user actually has permission to execute the shell)