libc, seconds: new time and date apis (try 2)

Redo date handling in libc almost entirely. This allows
handling dates and times from outside your timezones,
fixes timezone loading in multithreaded applications,
and allows parsing and formatting using custom format
strings.

As a test of the APIs, we replace the formatting code in
seconds(1), shrinking it massively.

The last commit missed a few removals, and made it
unnecessarily hard to do an update.

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