touch.1: Update to conform to POSIX 2004

POSIX borrowed the "double leap second" bug from C89. Double leap seconds can
never happen. This mistake was present in at least POSIX 1997 and fixed by
POSIX 2004. I can't find a copy of 2001 online to determine if the bug was
present in that revision.

While here, remove duplicate language between -d and -t. A few other minor
enhancements and an igor (lint) bugfix.

Further reading:

2018 POSIX (documents -d):
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/touch.html

2004 POSIX (documents SS from 0-60):
http://pubs.opengroup.org/onlinepubs/009695399/utilities/touch.html

1997 POSIX/SUSv2 (historical interest, 0-61):
http://pubs.opengroup.org/onlinepubs/007908799/xcu/touch.html

More on this subject (start at "Unix system time and the POSIX standard")
https://www.ucolick.org/~sla/leapsecs/onlinebib.html

And: https://marc.info/?l=openbsd-tech&m=92682843416159&w=2

Reported by: Vishal Sahu <vsahu AT isilon.com>
Sponsored by: Dell EMC Isilon