timezone daylight saving time, int16_t or boolean

It looks like `tz_dsttime` was originally designed to hold the offset value in seconds, but is used as a boolean.
For bluetooth (current time service), it would be better to use tz_dsttime as a second-offset, as bluetooth can have DST of 0, 30min, 60min, 120min [1]

os_time.h
~~~
struct os_timezone {
int16_t tz_minuteswest;
int16_t tz_dsttime;
};
~~~

datetime.c:
~~~
tz->tz_dsttime ? 3600 : 0
~~~

[1] https://www.bluetooth.com/specifications/gatt/viewer?attributeXmlFile=org.bluetooth.characteristic.dst_offset.xml