datetime: parse timezone abbreations

Use our own list of generated timezone names for symbol table
initialization, and then use this symbol table for datetime suffix
parsing without format.

Also, use newly integrated timezone names lookup functions in the
`tnt_strptime` if `%Z` flag was ordered. Return corresponding
`tm_gmtoff` (datetime->tzoffset) and `tm_tzindex`
(datetime->tzindex) for later consumption.

At the moment we could test only non-ambiguous abbreviations,
which might be converted to tzoffset in a deterministic way.

i.e. 'MSK' is always +0300, or 'MSD' is always +0400, but 'AT'
may be either 'Alaska Time' (-0900) or 'Atlantic Time' (-0400/-0300)
thus there is no (yet) any simple way to translate to their offset.

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