enableremote: Disallow using type= to attempt to change the type of an existing remote

enableremote: Disallow using type= to attempt to change the type of an existing remote

Changing the type out from under an existing special remote exposes the
existing config to something that may interpret it wildly differently. As
seen in the bug report, this can even result in behavior that makes
git-annex say it's buggy. So prevent the user from doing this. --sameas is
the better way.

Sponsored-by: Kevin Mueller