Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el: Fix defcustom type (char -> character).
nnimap.el (nnimap-open-connection): Remove %s from openssl incantation, which is no longer valid.
gnus-sum.el (gnus-summary-refer-thread): Implement a version that uses *-request-thread.
gnus-int.el (gnus-request-thread): New back end function.
nnimap.el (nnimap-request-thread): New back end function.
shr.el: Indent.
gnus-art.el, shr.el: Have shr switch buffer truncation on if there are big tables.
(nnimap-open-connection): Message when opening connection for debugging purposes.