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