Simplify use of current-time and friends

Simplify use of current-time and friends

* gnus-delay.el (gnus-delay-article):
* gnus-sum.el (gnus-summary-read-document):
* gnus-util.el (gnus-seconds-today, gnus-seconds-month):
* message.el (message-make-expires-date):
Omit unnecessary call to current-time.
* gnus-util.el (gnus-float-time): Simplify to an alias because
time-to-seconds now behaves like float-time with respect to nil arg.
(gnus-seconds-year): Don't call current-time twice to get the current
time stamp, as this can lead to inconsistent results.
* time-date.el (time-to-seconds) [!float-time]:
Use current time if arg is nil, to be compatible with float-time.
(time-date--day-in-year): New function, with most of the guts of
the old time-to-day-in-year.
(time-to-day-in-year): Use it.
(time-to-days): Use it, to avoid decoding the same time stamp twice.