Fix invisible erc-speedbar cursor in text terminals

Fix invisible erc-speedbar cursor in text terminals

* lisp/erc/erc-speedbar.el
(erc-speedbar--reset-last-ran-on-timer): Use `with-current-buffer'
instead of `setf' and `buffer-local-value'.
(erc-nickbar-mode, erc-nickbar-enable, erc-nickbar-disable)
(erc-speedbar-toggle-nicknames-window-lock): Revise doc string.
(erc-speedbar-toggle-nicknames-window-lock): Set `cursor-type' in
speedbar buffer.
(erc-nickbar-toggle-nicknames-window-lock): New function alias.  Note
that this name flouts traditional library namespacing conventions.
* lisp/erc/erc-status-sidebar.el (erc-status-sidebar-get-window): Use
`cursor-type' instead of `internal-show-cursor'.  (Bug#63595)