bug#49245: Enchant dictionaries list not being correctly set, and other minor fixes

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 27 Jun 2021 22:20:10 +0100
> From:  Reuben Thomas via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I found a bug recently where, when using Enchant as the back-end for ispell.el, spellchecking was locking up
> for some languages.
> 
> This turned out to be the combination of two problems.
> 
> First, ispell-find-enchant-dictionaries was incorrectly merging ispell-dictionary-base-alist into its result. This
> caused the ispell-set-spellchecker-params to fail to add the correct "-d LANG" flag arguments to the list of
> dictionaries that it used to start Enchant, which in turn meant that the process was started as e.g.
> 
> enchant-2 -d francais # rather than -d fr_FR
> 
> and failed to start properly. I have fixed this by simply removing the incorrect code (patch 0003 attached).
> 
> Secondly, ispell.el failed to notice that it had not actually started an Enchant process, and hung while trying to
> read from it in ispell-accept-output. I fixed this by testing that the process is live before trying to read from or
> write to it (patch 0002 attached).
> 
> Finally, while reading the source code I found an ancient comment that is more of a commit message in
> spirit (it explains how the current code came to be that way, rather than explaining something about how it
> works), so I removed it (patch 0001 attached).
> 
> As usual with my infrequent patches, I would appreciate other eyes on them before I install them, if possible.

OK for the first 2 patches, but please don't remove that comment, it
explains something important, and there's no reason to remove it (or
many other similar comments we have throughout our code).

Thanks.