gpx2gpsman: new -o option
new module Time::Zone::By4D

With test file.
GPS::Gpsman::Waypoints: new time-related methods

New methods unixtime_to_Comment and unixtime_to_DateTime.
Also in GPS::GpsmanData: helper function _unixtime_to_gpsmantime.

Along with new test file gps-gpsman-waypoint.t.
GPS::GpsmanData::Any: gpx conversion using new unixtime_to_* methods

This is the prerequisite for having settable timezone.
GPS::GpsmanData::Any: timeoffset support for gpx files
GPS::GpsmanData::TestRoundtrip with timeoffset support, too

And many more test cases in gps-gpsmandata-any.t.
GPS::GpsmanData::Any: support for timeoffset => automatic

--timeoffset option is now also available in gpx2gpsman script.
GPS::GpsmanData::TestRoundtrip: better roundtrip check

Now the intermediate gpsman file is explicitly saved to and loaded
from disk, so the final as_gpx step cannot take any "advantages" from
the formerly loaded GPS::GpsmanData object. With this change a problem
with timeoffset in segmented tracks became visible.
GPS::GpsmanData::Any: fix problem with timeoffset in track segments

Now all track segments get the correct timeoffset, not only the first
one. With test case.