Bring in :require and :save-function support.

Bring in :require and :save-function support.

* auth-source.el (auth-source-search): Add :require parameter, taking a
list.  Document it and the :save-function return token.  Pass :require
down.  Change the CREATED message from a warning to a debug statement.
(auth-source-search-backends): Pass :require down.
(auth-source-netrc-search): Pass :require down.
(auth-source-netrc-parse): Use :require, if it's given, as a filter.
Change save prompt to indicate all modifications saved here are
deletions.
(auth-source-netrc-create): Take user login name as default in user
prompt.  Move all the save functionality to a lexically bound function
under the :save-function token in the returned list.
(auth-source-netrc-saver): New function, intended to be wrapped for
:save-function.

* nnimap.el (nnimap-credentials): Keep the :save-function as the third
parameter in the credentials.
(nnimap-open-connection-1): Use it after a successful login.