Allow cancelling articles that have a differing From address

* gnus-msg.el (gnus-summary-cancel-article): `user-mail-address' is
buffer-local in some buffers, so bind it explicitly in the buffer we're
trying to cancel the article in (bug#10808).
ChangeLog belonging to the previous checkin
epa.texi (Mail-mode integration): Mention epa-mail-aliases
Address parsing fixup

* ietf-drums.el (ietf-drums-parse-address): Don't bug out when called
with an empty string.
Make it possible to specify servers to be covered by the cloud
Make it possible to specify servers to be covered by the cloud
(gnus-server-toggle-cloud-server): Only allow clouding applicable types.
nnimap.el (nnimap-split-rule): "Use instead" obsolescence messages should end with "."
gmm-utils.el (gmm-flet, gmm-labels): Add edebug spec
gnus-art.el (gnus-mime-buttonize-attachments-in-header): Display buttons that are hidden in unselected alternative part as well
auth.texi (GnuPG and EasyPG Assistant Configuration): Be agnostic about authinfo/authinfo.gpg default order (Bug#16642)
auth-source.el (auth-sources): Doc fix (Bug#16642)