homectl: when chainloading a shell, prefix "-" rather than overriding first char

homectl: when chainloading a shell, prefix "-" rather than overriding first char

Login shells are supposed to marked via a dash as first char. We follow
that logic, but right now we simply overwrite the first char of the
shell. That might not be the right choice, given that this turns
"zsh" into "-sh", which suggests some bourne shell process.

Hence, let's correct things, and instead prefix a dash, which should be
safer.

Inspired by findings on https://github.com/systemd/systemd/issues/34153#issuecomment-2338104907