Make erc--server-reconnecting non-buffer-local

Make erc--server-reconnecting non-buffer-local

* lisp/erc/erc-backend.el (erc--server-reconnecting): Mention expected
non-nil value type in doc string.
(erc-server-connect): Don't set `erc--server-reconnecting'.
(erc-server--reconnect): Let-bind `erc--server-reconnecting' instead
of setting it locally in the server buffer.  Set it to an alist
containing the current buffer's local variables.
(erc-process-sentinel-2): Don't set `erc--server-reconnect'.
* lisp/erc/erc.el (erc--cmd-reconnect): Clean up some assertions.
(Bug#57955.)