Refactor new --sshcopyid behavior into separate cmdline option --sshcopyauth

Related to commit 07e835eac7 and the discussion within
https://github.com/grml/grml-debootstrap/pull/153

If execution of --sshcopyid fails, then user might want to be aware of
it. So instead of implementing the copying of .ssh/authorized_keys as
fallback of --sshcopyid, let's provide it via cmdline option
--sshcopyauth.

Reviewed-by: Chris Hofstaedtler
Reviewed-by: Darshaka Pathirana
Closes: https://github.com/grml/grml-debootstrap/pull/153