title: Add authorized key with only one SSH login
body: <p>The typical procedure for a user to add an authorized key to <code>~/.ssh/authorized_keys</code> is:</p>
  
  <ol>
  <li>Copy the public key on the remote host with <code>scp</code>.</li>
  <li>Connect with <code>ssh</code> and add the key to the file.</li>
  </ol>
  
  <p>Such a procedure is for instance described in an answer to ”<a href="https://stackoverflow.com/q/17400719/3990969">How to add a ssh key to remote server?</a>”</p>
  
  <p>Is this possible to add the key in only one step — with only one login to the remote server? An answer working on FreeBSD and Debian GNU/Linux (both with OpenSSH) would be great.</p>