- SCP and SFTP with the multi interface had the same flaw: the 'DONE'

  operation didn't complete properly if the EAGAIN equivalent was returned but
  libcurl would simply continue with a half-completed close operation
  performed. This ruined persistent connection re-use and cause some
  SSH-protocol errors in general. The correction is unfortunately adding a
  blocking function - doing it entirely non-blocking should be considered for
  a better fix.