Re-haul logger hierarchy

1. Allow choosing between system clock and local time (the default) by defining LOGGER_USE_LOCAL_TIME_
2. The path formatting string(s) passed to C_PathFmtLogSnap::configPath() is now used as fmt parameter of std::vformat() instead of format parameter of std::strftime()
3. Remove class C_UseLogger and extract the essence into more reusable function stamp()
4. Macro DEF_FALLBACKABLE_LOGGER_FILES() renamed to shorter DEF_FALLBACK_LOGGER_FILES()
5. Replace -g3 -Og with -O2 to build more efficient library.