Reuse client cert for connectivity probing in ERC

Reuse client cert for connectivity probing in ERC

* lisp/erc/erc-backend.el (erc--server-connect-function)
(erc--server-post-dial-function): Rename former to latter because the
existing name is better suited for the eventual generalizing of
`erc-server-connect' in a future version.
(erc-server-connect): Use new name for `erc--server-connect-function',
`erc--server-post-dial-function'.
(erc--recon-probe-reschedule, erc--recon-probe-sentinel)
(erc--recon-probe-filter, erc--recon-probe-check): New functions
factored out of `erc-server-delayed-check-reconnect'.
(erc-server-delayed-check-reconnect): Refactor, splitting off lambdas
into top-level functions for improved tracing.
* lisp/erc/erc.el (erc-message-english-recon-probe-hung-up)
(erc-message-english-recon-probe-nobody-home): New
variables.  (Bug#62044)

Thanks to Libera.Chat user arjan for reporting this bug, which
is new in ERC 5.6 and Emacs 30.1.

(cherry picked from commit 331bcfaee51f7fa5ff0f6046f30e940452f3a8fe)