Adds a newline at the very end of the authorized_keys file

* This is important because it allows new keys to be added via `cat ...
  >> ~/.ssh/authorized_keys` or `echo ... >> ~/.ssh/authorized_keys`
  Without a newline at the end of the file `>>` will concat text to the
  end of the last key, mangling the file.
* These modifications should handle files that currently do *and* do not
  have newlines at the very end, though it will always write files
  that *do* have newlines at the very end.