authneg: clear auth.multi flag at http_done

This flag is meant for the current request based on authentication
state, once the request is done we can clear the flag.

Also change auth.multi to auth.multipass for better readability.

Fixes https://github.com/curl/curl/issues/1095
Closes https://github.com/curl/curl/pull/1326



Signed-off-by: Isaac Boukris <iboukris@gmail.com>
Reported-by: Michael Kaufmann