Merge changes made in Gnus master

Merge changes made in Gnus master

2012-10-05  Katsumi Yamaoka  <yamaoka@jpl.org>
 * gnus.texi (Mail Source Specifiers):
 Document :leave keyword used for pop mail source.
2012-10-25  Tassilo Horn  <tsdh@gnu.org>
 * gnus-dired.el (gnus-dired-attach): Attach to last used message buffer
 by default.  Patch provided by Stephen Eglen.
2012-10-05  Katsumi Yamaoka  <yamaoka@jpl.org>
 New UIDL implementation.
 * mail-source.el (mail-sources, mail-source-keyword-map):
 Add :leave as a pop3 keyword.
 (mail-source-fetch-pop): Bind pop3-leave-mail-on-server.
 * pop3.el (pop3-leave-mail-on-server): Allow number.
 (pop3-uidl-file, pop3-uidl-file-backup): New user options.
 (pop3-movemail): Add UIDL support.
 (pop3-send-streaming-command): Take a list of mail numbers instead of
 the number of mails.
 (pop3-write-to-file): Add X-UIDL header.
 (pop3-uidl-stat, pop3-uidl-dele, pop3-uidl-load, pop3-uidl-save)
 (pop3-uidl-add-xheader): New functions.
 * message.el (message-ignored-resent-headers):
 Add X-Content-Length and X-UIDL headers.