Reopening.

This bug's original patch set tried to walk back an austere aspect of
bug#51753's changes that lumped interactive buffer creation in with
protocol driven creation. It mainly did so by introducing a new option,
`erc-interactive-display', that reinstated direct window-buffer
replacement when running M-x erc. However, as pointed out by Corwin on
Libera, this did not account for interactive /JOINs.

The attached patch aims to address this oversight as well as simplify
the display-options picture a bit further by consolidating the roles of
`erc-interactive-display' and `erc-query-display'. The thinking is that
both options cover the same basic ground involving buffers created as a
consequence of issuing slash commands. This patch also adds an interface
for other such commands to use for detecting when they're being called
from the command line.