Issues with local time

[ianchi]

1. The "local time" shows the time in the browser's TZ instead of in the router's TZ.
2. There is no ubus call to set the time

The problem is that ubus system.info.localtime needs conversion to string, and when done in the browser, it uses its TZ.

@jow- I think we could add a new property localtimestr to system.info.
I couldn't find which package provides ubus methods for the system namespace.

To which namespace shoud we add a set_time method? rpc-sys? system?
In the meantime I'll add it to luci2.system