Adjust date/time functions so that they do a better job of keeping track

Adjust date/time functions so that they do a better job of keeping track
of whether the current time is UTC or localtime, and no-op the 'utc' and
'localtime' modifiers accordingly. See
[forum:/info/e7a939e074|forum post e7a939e074].
Also add the datedebug() function, available
only under -DSQLITE_DEBUG, for improved visibility of the DateTime object
during debugging and testing.

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