sshcopyid option: exit if `ssh-add -L` fails to execute

If a user requested the --sshcopyid option then it's supposed
to be run successfully, otherwise it might leave an inaccessible
system, so exit with error if execution of `ssh-add -L` fails.

While at it:

* simplify code (no need for $RC)
* don't fail if directory /root/.ssh exists already
* append keys to possibly existing /root/.ssh/authorized_keys
  file, instead of overwriting it