Improve gnus thread-referral

Improve gnus thread-referral

Allow thread referral to use search whenever possible, displaying the
results in the current summary buffer if possible and a new nnselect
buffer if not.

* lisp/gnus/nnimap.el (nnimap-request-thread): Obsolete function.
* lisp/gnus/gnus-search.el (gnus-search-thread): Allow detailed
specification of how/where to search. Add found articles to the
current summary buffer if possible, or create a new ephemeral nnselect
group if not.
* lisp/gnus/gnus-sum.el (gnus-refer-thread-use-search): Allow a list
of servers and groups to search.
(gnus-summary-refer-thread): Find thread-related articles by using a
backend-specific method, gnus-search, or retrieving nearby headers in
the current group.
* lisp/gnus/nnselect.el (nnselect-search-thread): Obsolete function.
(nnselect-request-thread): Allow thread referral from nnselect groups.
* doc/misc/gnus.texi (Finding the Parent): Document changes to thread
referral.