Overhaul `datetime()` scene language function.

[c-lipka]

• Added local_time BOOL global setting, defaulting to off; when set to on, datetime() will now report local time instead of UTC.
• datetime() will now sanity-check the specified format string (unless the default is used), and generate various warnings if applicable.
• datetime() will now try to distinguish between a genuine error, a and non-error situation in which the result string just happens to be empty.
• LC_TIME locale is now set someplace more sane.

Also, pushed back on the use of boost date_time library in favor of C++11 features.