core: change/recover in-use nick; fix obsolete USER command syntax

[Exirel]

Description

Basic implementation to fix #300 (no configuration, no alternative, nothing):

1. on connection, if we receive a "nick already used" event, we change the current nick, adding an extra _ to its end
2. on auth with a nickserv account, if bot.nick is not bot.settings.core.nick, we try to use NickServ GHOST
3. when bot.settings.core.nick QUIT, we try to get back to that nick

Maybe this should be optional, maybe we could have a list of nick aliases to use for that purpose. But I wanted to show what is possible with the current state of the code. It tested it by using a copy of a config file, and checking that the second instance ghost the first one and get its nick back.

From another user point of view, here are the logs:

15:25:05 → Exibot joined (~Exibot@hostmask)
15:27:33 → Exibot_ joined (~Exibot@hostmask)
15:27:33 ⇐ Exibot quit (~Exibot@hostmask) Disconnected by services
15:27:34 Exibot_ → Exibot

After some discussions, I also fixed an improper use of the USER command, and made some minor code-style changes.

I'll do an Exirel and say: everything else is good for another PR!

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[Exirel]

Note that it should fix #300 which is the current oldest issue we have for now.

[dgw]

Found a few nemeseS to slay. Despite a bit of doom and gloom in a couple of my line notes, this seems like a good approach. I'm just a bit mad at shortcomings of the IRC protocol and/or Sopel's threaded/async architecture. 😅

One other thing I couldn't touch in line notes because of GitHub's arbitrary restrictions:

./sopel/irc/__init__.py:49:1: F401 'sopel.tools.events' imported but unused

[dgw]

I'll probably forget to include the USER fix in release notes, but that's what updating PR titles is for. 🤪