Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-group.el (gnus-group-mark-article-read): It's possible that we want to have `gnus-newsgroup-unselected' kept sorted.  If this isn't done, then unselected articles may be marked as read.
pop3.el (pop3-open-server): Erase the buffer after the greeting, since not doing this seems to lead to a race condition in pop3-logon.
nnvirtual.el (nnvirtual-request-article): Bind `gnus-command-method' so that the call chain it correct when we call "upwards".
gnus-sum.el (gnus-select-newsgroup): Auto-expiry doesn't make sense in read-only groups.
gnus-group.el (gnus-group-mark-article-read): Ditto.
message.el (message-cite-reply-position): Doc string fix.
nnimap.el (nnimap-transform-headers): Simplify regexp to hopefully avoid regexp overflow.
 (nnimap-transform-split-mail): Ditto.
pop3.el (pop3-retr): Error out if the server closes the connection.