bug#67032: 30.0.50; ERC 5.6-git: Treat erc-send-message more responsibly

[J.P.]

> This has been installed as
>
>   1d2aa130cae * Revive erc-command-indicator as new module

One aspect this change should've addressed but didn't was using the
revived `command-indicator' facility in `erc-load-irc-script-lines' to
make its output more predictable and consistent with other displayed
messages. While leaving it to rot would of course be safer, I suspect
it's rarely used enough to warrant the risk. An ancillary benefit of
fixing this is that it helps clarify the role of boolean return values
for `erc-process-input-line', `erc-send-input-line-function',
`erc-message', and the various `erc-cmd-FOO' slash-command handlers.
See second patch (lacks tests).

Thanks.

P.S. The commit named above unfortunately cites the wrong bug number
(67031) in its commit message.

0001-5.6-Restore-leading-space-to-right-margin-stamps-in-.patch
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0002-5.x-Use-command-indicator-in-erc-load-irc-script-lin.patch
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