Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

message.texi (Message Buffers): Update default value of message-generate-new-buffers.
shr.el (shr-table-horizontal-line): Change the defaults for the table lines to be spaces instead.
sieve-manage.el (sieve-sasl-auth): Create auth-info if not found.
 (sieve-sasl-auth): Check that auth-source-search did return something, or just return an empty string.
gnus-start.el (gnus-group-change-level): Allow putting foreign groups onto the list of killed groups, too.  This makes killed nnimap groups, for instance, more reliably not reappear.
nnimap.el (nnimap-request-thread): Don't bug out when we can't find the parent.