(posix_date): Remove; not needed as a static var.

(posix_date): Remove; not needed as a static var.
All uses rewritten.
(touch): Use new futimens function to operate more efficiently
in some cases.  Don't stat/fstat existing file when
(!amtime_now && change_times == (CH_ATIME | CH_MTIME)); the
old time stamps aren't needed in that case.
(main): change_times is int, not bool.  Simplify test for
change_times.