Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus.texi (Subscription Methods): Link to "Group Levels" to explain zombies.  (Checking New Groups): Ditto (bug#8974).
gnus.texi (Startup Files): Clarify that we're talking about numbered backups, and not actual vc (bug#8975).
gnus-group.el (gnus-read-ephemeral-bug-group): Bind the coding systems to binary before writing and reading the mbox files.
gnus.el (gnus-summary-line-format): Link to the info node for %U instead of trying to list them all (bug#8978).
pop3.el (pop3-open-server): Use :end-of-capability.
gnus-group.el (gnus-read-ephemeral-emacs-bug-group): Make sure that the id is always a number.
gnus-group.el (gnus-read-ephemeral-emacs-bug-group): Hook into debbugs mode, if possible.