utmpdump: use always UTC-0 timezone in textual output

utmpdump: use always UTC-0 timezone in textual output

Converting a time structure from text format that has timezone markup is
practically impossible.  See reference links for more information.  This
leads to situation where multiple utmpdump(1) conversions from binary to
text and back make timestamps to shift amount of timezone offset to UTC-0.

The easiest way to make multiple conversions to work without timeshifts is
to always use UTC-0 timezone.  Downside of this approach is that the textual
format is less human readable than local timestamps would be.

Reference: http://www.catb.org/esr/time-programming/#_strptime_3_and_getdate_3
Reference: http://man7.org/linux/man-pages/man3/strptime.3.html
Signed-off-by: Sami Kerola <kerolasa@iki.fi>