v2. Respect `erc-prompt-for-password'. Remove `run-at-time' hack from
regression fix.

"J.P." <jp@neverwas.me> writes:

> The proposed patch abandons most of the envisioned niceties and
> instead employs a hack to just force a non-local exit and schedule a
> call to `erc-tls' when appropriate. It also instructs users to do that
> themselves in the future (issue an M-x erc-tls, that is). Hopefully,
> this makes for an acceptable compromise.

I've forsaken the first part out of an abundance of caution. So this now
just prints a warning and proceeds with a non-TLS connection. If anyone
finds that unsatisfactory, please say something.

> BTW, a POC patch set far more faithful to the original breadth of the
> feature (and way less hacky) is also attached. Perhaps it'll inform
> further exploration in this area down the road (but not for Emacs 29).

IOW, the de facto goal here is to make interactive `erc' a "universal"
entry point for both plain and TLS connections.