Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nntp.el (nntp-authinfo-file): Mark as obsolete -- use auth-source instead.
pop3.el (pop3-wait-for-messages): Don't use Gnus functions here.
gnus-util.el (gnus-process-live-p): Copy over compat function.
pop3.el (pop3-wait-for-messages): If the pop3 process dies, stop processing.
nntp.el (nntp-kill-buffer): Kill the process before killing the buffer to avoid warnings.
gnus-agent.el (gnus-agent-expire-done-message): Use %.f as format specified to reduce precision.