#56 Semantics of RTC frequency wrong on Linux

[Christoph]

Must return ticks per second as on Windows, because caller might perform own conversion of native value.

[Christoph]

I think I have fixed that, but I am not sure any more. Could someone check:

    switch (performance_counter::check_rtc_availability()) {
        case has_rtc_type::yes:
            // clock_gettime returns nanoseconds.
            performance_counter::frequencyValue
                = performance_counter::nanoseconds_per_second;
            break;

        case has_rtc_type::no:
            // gettimeofday returns microseconds.
            performance_counter::frequencyValue
                = performance_counter::microseconds_per_second;
            break;

        default:
            // This should be unreachable.
            THE_ASSERT(false);
            break;
    }