NetworkAgent: disallow multiple requests for the same connection/setting

NetworkAgent: disallow multiple requests for the same connection/setting

It doesn't make sense to have multiple requests for the same
connection/setting combination at the same time, since we would be
asking the user twice for the same password. Instead, report cancellation
to NetworkManager if this happens.
Note that does make sense to have multiple requests in sequence though
(they could have different flags), but this is not affected.

https://bugzilla.gnome.org/show_bug.cgi?id=674961