Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

auth.texi (Help for users): Mention ~/.netrc is also searched by default now.
gnus-start.el (gnus-dribble-read-file): Set buffer-save-without-query, since we always want to save the dribble file, probably.
nnmail.el (nnmail-article-group): Allow a final "" split to work on nnimap.
gnus-sum.el (gnus-user-date-format-alist): Renamed back again from -summary- since it's a user-visible variable.
nnimap.el (nnimap-retrieve-group-data-early): Don't do QRESYNC the first time you use the new Gnus.
auth-source.el: Don't load netrc.el.
 (auth-sources): Search ~/.netrc as well by default.
 (auth-source-debug): Add 'trivia option for extra output.
 (auth-source-do-trivia): Use it.
 (auth-source-search): Simplify logic to use `auth-source-search-backends'.  Use `auth-source-do-trivia' where appropriate.  Don't keep a running count at this level.  Layer :create and :delete options appropriately on the first and second passes.  Don't track the backend with the search results.
 (auth-source-search-backends): New function to search a list of backends for a processed spec.
 (auth-source-netrc-parse): Cache all netrc files, making auth-source-netrc-cache an alist keyed by the file name and using the file mtime as the caching criterion.  Keep the obfuscated data secret with a lexical bind.
 (auth-source-netrc-search): Don't calculate the length of the results unnecessarily.
 (auth-source-search-backends): Fix bug.
 (auth-source-netrc-create): Rework prompts.
nnir.el (nnir-imap-search-arguments,nnir-imap-default-search-key): Lower case names of search constraints.
 (nnir-run-query): Cache and reuse search constraints for all imap servers.
gnus-msg.el (gnus-setup-message): Define missing variable from last checkin.