Use :port instead of :protocol for all auth-source functions.

Use :port instead of :protocol for all auth-source functions.

* nnimap.el (nnimap-credentials): Pass a port default to
`auth-source-search' in case an entry needs to be created.
(nnimap-open-connection-1): Use :port instead of :protocol.

(auth-source-netrc-search, auth-source-netrc-create)
(auth-source-secrets-search, auth-source-secrets-create)
(auth-source-user-or-password, auth-source-backend, auth-sources)
(auth-source-backend-parse-parameters, auth-source-search): Use :port
instead of :protocol.

* auth-source.el (auth-source-netrc-parse): Use :port instead of
:protocol and accept a missing user, host, or port as a wildcard match.