Add proper support for time restrictions which wrap around a day

[Syntax505]

Time restrictions which wrap around a day (e.g. 2300 - 1159z) are presently not implemented correctly, as VFPC would continue to output "OK" for such a restriction even if the current UTC time falls within that range. This PR resolves that issue.

[EightSmart]

I would pull out the time range check logic to avoid duplication and make the code neater

bool withinPeriod;
if (startTimeGreater) {
    withinPeriod = (tm_gmt->tm_hour >= time["start"].get<int>() ||
                    tm_gmt->tm_hour <= time["end"].get<int>());
} else {
    withinPeriod = (tm_gmt->tm_hour >= time["start"].get<int>() &&
                    tm_gmt->tm_hour <= time["end"].get<int>());
}

if (withinPeriod) {
    // Loop through available levels
}

[EightSmart]

Also you don't need the if statement to assign the previous bool:
bool startTimeGreater = time["start"].get<int>() > time["end"].get<int>();