Normalize usage of variable erc-server-reconnecting

Normalize usage of variable erc-server-reconnecting

* lisp/erc/erc-backend.el (erc-server-reconnecting-p):
Don't consider erc-server-reconnecting when rendering verdict.
(erc-process-sentinel-2): ensure local variable
erc-server-reconnecting is t when timers are scheduled or running and
trying to reconnect, and nil otherwise, including after no retries
remain. Previously, its use and meaning in erc-backend were convoluted
and conflicted with its use in erc-cmd-RECONNECT (bug#50007).