Deprecate instead of redefine erc-server-reconnecting

Deprecate instead of redefine erc-server-reconnecting

* lisp/erc/erc-backend.el (erc-server-reconnecting,
erc--server-reconnecting): obsolete and replace the former with new
internal variant, which carries a simplified meaning.

(erc-server-reconnect-p, erc--server-reconnect-p): Obsolete and
replace the former with an internal function, and change behavior to
disregard `erc-server-reconnecting' when rendering verdict.

(erc-process-sentinel-2): ensure local var `erc--server-reconnecting'
is t when timers are scheduled or firing, and nil otherwise, including
after retries exhausted.  This agrees with the straightforward way
`erc-server-reconnecting' has always been used by `erc-cmd-RECONNECT'.

(erc-server-connect): set `erc--server-reconnecting'.

* lisp/erc/erc.el (erc-cmd-RECONNECT): use `erc--server-reconnecting'
instead of `erc-server-reconnecting'.