sleep: Overhaul.

sleep: Overhaul.

Program:

* Add a dummy getopt(3) loop to handle `--`.
* Move interval parsing out into a separate function.
* Print a diagnostic for every invalid interval.
* Check for NaN and infinity.
* Improve bounds checks.

Manual page:

* Miscellaneous markup fixes.
* Reword DESCRIPTION section.
* Move text about GNU compatibility to STANDARDS section.
* Convert examples from csh to sh.

Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D44471