Port new time stamp handling to old Emacs and to XEmacs.

Port new time stamp handling to old Emacs and to XEmacs.

This is needed for Gnus, which copies time-date.el and which
runs on older Emacs implementations.
* calendar/time-date.el (with-decoded-time-value):
Handle 'nil' and floating-point arg more compatibly with new Emacs.
(encode-time-value, with-decoded-time-value):
Obsolete only if new Emacs.
(time-add, time-subtract, time-less-p): Define if not new Emacs.