warn and refuse to autoenable a special remote when name is in use

warn and refuse to autoenable a special remote when name is in use

Improve behavior when there are special remotes configured with
autoenable=yes with names that conflict with other remotes.

The use of remoteList' is to avoid using the cached remote list in the case
where there are two special remotes both configured to autoenable and both
with the same name. Once the 1st is autoenabled, this makes reload the
remote list and so see the 1st, and so refuse to autoenable the second.

This adds a little bit of overhead, but it should be sufficiently small not
to need optimising.

Sponsored-by: Dartmouth College's OpenNeuro project