Do smarter auth-source creation if needed and still work without creation.

Do smarter auth-source creation if needed and still work without creation.

* nnimap.el (nnimap-credentials): Remove the `inhibit-create'
parameter.  Create entry if necessary by using :create t.
(nnimap-open-connection-1): Don't pass `inhibit-create'.

* auth-source.el (auth-source-netrc-create): Return a synthetic search
result when the user doesn't want to write to the file.
(auth-source-netrc-search): Expect a synthetic result and proceed
accordingly.