fix sign of strftime %z output with offsets <1 hour west of UTC

fix sign of strftime %z output with offsets <1 hour west of UTC

the sign character produced came from the sign of tm_gmtoff/3600 as an
integer division, which is zero for negative offsets smaller in
magnitude than 3600. instead of printing the hours and minutes as
separate fields, print them as a single value of the form
hours*100+minutes, which naturally has the correct sign.