bootctl: decouple "list", "unlink", "cleanup"

bootctl: decouple "list", "unlink", "cleanup"

These operations to quite different things, they just share 2 common
funcs. Let's split them out into separate files.

This also splits up verb_list() into separate calls for the three
operations. This actually fixes issues, as for status/list we want
"unpriv" ESP discovery logic, but for the other two we really should
have privileged discovery logic.

This is preparation for adding "bootctl link" later, but this makes
sense either way, I am sure.