Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-picon.el (gnus-picon-create-glyph): Set the background XPM colour explicitly.
message.el (message-cite-prefix-regexp): Remove } from the cite prefix.
gnus-win.el (gnus-window-to-buffer-helper, gnus-all-windows-visible-p): Function needn't be a symbol.
mail-source.el (mail-source-value): Function needn't be a symbol.
tls.el (open-tls-stream): Don't query killing process.
gnus-win.el: Revert previous patch, since it made Gnus backtrace.
nnimap.el: Look up IMAP credentials based on both the virtual and physical server names.
mail-source.el (mail-source-value): Revert previous patch.