Rename tm_gmtoff to tm_utcoff

Rename the `tm_gmtoff` field of `time::Tm` to `tm_utcoff`.

Although GMT and UTC are commonly used interchangeably, UTC is more appropriate to denote the time standard.

Python, for example, uses `datetime.utcoffset()` to represent the offset from UTC.

I think we should fix this while we are allowed to break backward compatibility.