Adopt clock features used by core into core

[dgw]

User/channel time zone/format preferences shouldn't live in a plugin. Those preferences are used by core API features, most notably the tools.time subpackage, and many other first- & third-party plugins rely on them to correctly format their output.

The functions to set/get/clear time zone & time format preferences for both users and channels probably should live in coretasks.

As I wrote in my do-something-about-this-someday text note: "[These clock features that couple to core API functions] are almost as bad as when disabling [the url plugin] broke the @url decorator."

---

I think I'd like to split up coretasks and compartmentalize protocol handling from always-available core commands like .blocks and the time-preference stuff I'm proposing to adopt here, but that's a separate discussion.