nnimap.el (nnimap-unselect-group): Made into its own function.

nnimap.el (nnimap-unselect-group): Made into its own function.
 (nnimap-request-rename-group): Unselect group before renaming.  This had gotten lost somewhere.
 (nnimap-request-accept-article): Keep track of examined groups, and unselect the group before APPENDing to read-only groups.
 (nnimap-request-move-article): Clear flags before moving so that they can be re-set later.