* gnus-group.el (gnus-group-name-at-point): Rewrite; rename from

* gnus-group.el (gnus-group-name-at-point): Rewrite; rename from
 group-name-at-point.
(gnus-group-completing-read): New function that offers decoded non-ASCII group
 names for completion.
(gnus-fetch-group, gnus-group-read-ephemeral-group)
(gnus-group-jump-to-group, gnus-group-make-group-simple)
(gnus-group-unsubscribe-group, gnus-group-fetch-charter)
(gnus-group-fetch-control): Use it.
(gnus-fetch-group): Use group-name-at-point for the initial value rather than
 the default value; use gnus-alive-p.

* gnus-msg.el (gnus-group-mail, gnus-group-news, gnus-group-post-news)
(gnus-summary-mail-other-window, gnus-summary-news-other-window)
(gnus-summary-post-news): Use gnus-group-completing-read.

* gnus-sum.el (gnus-select-newsgroup): Decode group name in error msg.
(gnus-read-move-group-name): Decode group name for completion.