Using seconds offset from GMT to local time to compute local time give
errors when printing times outside the same daylight saving time period.
Use the localtime() function for default and -tttt cases.
Use the gmtime() function for -ttt and -ttttt cases.
Rename ts_hmsfrac_print() to ts_date_hmsfrac_print().
Remove some variables now useless.
Update some comments.
The structure 'netdissect_options' can hold if_printer.
Thus the structure 'print_info' is useless now.
Moreover:
Delete useless blank lines
Fix indent
