Default to TLS port when calling erc-tls from lisp

Default to TLS port when calling erc-tls from lisp

* lisp/erc/erc.el (erc-normalize-port): Add standard IANA port-name
mappings for 6667 and 6697.
(erc-open): Add note to doc string explaining that params `connect'
and `channel' are mutually exclusive.
(erc-tls): Call `erc-compute-port' with override.
(erc-compute-port): Call `erc-normalize-port' with result'.
* test/lisp/erc/erc-tests.el (erc-tls): Add simplistic test focusing
on default parameters.  (Bug#56514.)