Remove duplicate ERC prompt on reconnect

Remove duplicate ERC prompt on reconnect

* lisp/erc/erc-backend.el (erc--unhide-prompt, erc--hide-prompt,
erc--unhide-prompt-on-self-insert): Add functions to ensure prompt is
hidden on disconnect and shown when a user types /reconnect in a
disconnected server buffer.
(erc-process-sentinel): Register aforementioned function with
`pre-command-hook' when prompt is deleted after disconnecting.
(erc-server-PRIVMSG): Ensure prompt is showing when a new message
arrives from target.

* lisp/erc/erc.el (erc-hide-prompt): Repurpose unused option by
changing meaning slightly to mean "selectively hide prompt when
disconnected."  Also delete obsolete, commented-out code that at some
point used this option in its prior incarnation.
(erc-prompt-hidden): Add new option to specify look of prompt when
hidden.
(erc-unhide-query-prompt): Add option to force-reveal query prompts on
reconnect.
(erc-open): Augment earlier reconnect-detection semantics by
incorporating `erc--server-reconnecting'.  In existing buffers, remove
prompt-related hooks and reveal prompt, if necessary.
(erc-cmd-RECONNECT): Allow a user to reconnect when already
connected (by first disconnecting).
(erc-connection-established): Possibly unhide query prompts.
(Bug#54826)

* test/lisp/erc/erc-tests.el (erc-tests--test-prep,
erc-tests--set-fake-server-process): Factor out some common
buffer-prep boilerplate involving user input and the server process.
Shared with bug#54536.