Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-sum.el (gnus-propagate-marks): Change default to t again, since nil means that nnimap doesn't get updated.
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.
 (auth-source-cache-expiry): New variable to override `password-cache-expiry'.
 (auth-source-remember): Use it.
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'.