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.

[breaking-change]
Merge pull request #1 from barosl/gmtoff-to-utcoff

Rename tm_gmtoff to tm_utcoff