Don't allow the type of encryption of an existing special remote to be changed.

Don't allow the type of encryption of an existing special remote to be changed.

eg, git-annex enableremote foo encryption=none will not remove encryption,
and other encryption= settings don't change the type of encryption used.
Either of which would render data stored in a special remote inaccessible.

Probably fixes reversion introduced in
71f78fe45dc91dbef0bedd79b33d6a9fed85704d.
That commit got rid of the hasEncryptionConfig check, which I think would
have detected this before. I've not gone back to verify that.

Sponsored-by: mycroft