Compare significant fields only for simple SSH keys

For simple SSH keys, that is those without options such as
“command="…"”, only the first two parts need to be compared. The third
field is a free-form comment.

This patch changes the comparison used in
AddAuthorizedKey/RemoveAuthorizedKey to take this into account. Lines
with options are still compared in full.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>