Rename Gnus -unsubscribe-group commands to -toggle-subscription

Rename Gnus -unsubscribe-group commands to -toggle-subscription

* doc/emacs/misc.texi (Gnus Group Buffer): Document change.
* doc/misc/gnus.texi (Subscription Commands): Document change.
(Browse Foreign Server): Document change.
* lisp/gnus/gnus-group.el (gnus-group-mode-map):
Unsubscribe is not subscribe.
(gnus-group-sub-map): Unsubscribe is not subscribe.
(gnus-group-make-menu-bar): Unsubscribe is not subscribe.
(gnus-group-tool-bar-gnome): Unsubscribe is not subscribe.
(gnus-group-mode): Unsubscribe is not subscribe.
(gnus-group-unsubscribe): Refactor.
(gnus-group-subscribe): Refactor.
(gnus-group-unsubscribe-current-group): Define obsolete alias.
(gnus-group-unsubscribe-group): Define obsolete alias.
(gnus-group-toggle-subscription-at-point): Refactor.
(gnus-group-set-subscription-at-point): Refactor.
(gnus-group-toggle-subscription): Refactor.
(gnus-group-set-subscription): Refactor.
* lisp/gnus/gnus-srvr.el (gnus-browse-mode-map):
Unsubscribe is not subscribe.
(gnus-browse-make-menu-bar): Unsubscribe is not subscribe.
(gnus-browse-mode): Document the change.
(gnus-browse-unsubscribe-current-group): Define obsolete alias.
(gnus-browse-unsubscribe-group): Define obsolete alias.
(gnus-browse-toggle-subscription-at-point): Unsubscribe is not subscribe.
(gnus-browse-toggle-subscription): Unsubscribe is not subscribe
(bug#49759).