Migrate from using bot.privileges to bot.channels (if available)

[dgw]

Lots of "if available" features these days.

Sopel 6.2.0 deprecated the bot.privileges dict. It was replaced by bot.channels, which contains more state information. Obviously this depends on #8 (but should not block it—see below).

As of 6.3.0 the bot.channels dict doesn't seem to be populated quite correctly, but when it's stabilized and can replace bot.privileges it should be used if available, as the deprecated API will probably disappear in 7.x (or by 8.x at the latest).

[dgw]

bot.channels is still bugged in 6.4.0, according to sopel-irc/sopel#1136.