Don't send erc-sasl-user as USER command argument

Don't send erc-sasl-user as USER command argument

* lisp/erc/erc-sasl.el (erc-sasl--send-cap-ls): Add internal switch
for sending an opening "CAP LS".  The rationale for not enabling this
by default is twofold: one, it more strongly implies that ERC supports
IRCv3 client capability negotiation, which is somewhat disingenuous;
and, two, We'd still be "faking it" by firing and forgetting, and more
balls in the air makes things less predictable.
(erc--register-connection): Possibly send a "CAP LS" before anything
depending on the value of `erc-sasl--send-cap-ls'.  Also, don't
attempt to send `erc-session-username' when it holds an SASL username
because the latter may contain protocol-defying characters.
* test/lisp/erc/resources/base/local-modules/fourth.eld: change user
parameter of "USER" command to reflect nick when `erc-sasl-user' is
set to `:user'.  (Bug#59976.)