Accept user keyword arg in ERC entry-point commands

Accept user keyword arg in ERC entry-point commands

* lisp/erc/erc-backend.el (erc-server-reconnect): Reuse the username
argument from the previous session's USER command when reconnecting.
Also pass the existing client certificate, fixing an issue related to
bug#47788.
(erc-session-user-full-name): Move variable here from erc.el.
(erc-session-username): Add new local variable to store entry point
parameter.

* lisp/erc/erc.el (erc-session-user-full-name): Move variable to
erc-backend.
(erc-open, erc-determine-parameters, erc, erc-tls): Accept new
optional user parameter.
(erc-query): Preserve current `erc-session-username' when calling
`erc-open'.
(erc-login): Use `erc-session-username' instead of deriving it.
(erc-compute-user): Add new function to determine user name from
explicit argument or user options.
(Bug#54824)