sleep.3: miscellanous cleanup, rewrites

sleep.3: miscellanous cleanup, rewrites

Highlights:

- Tighten up the NAME.
- "process" -> "thread".
- Tidy up the DESCRIPTION.  In particular, omit discussion of the
  historial SIGALRM-based implementation.
- Simplify RETURN VALUES.
- Add an ERRORS section, note that sleep(3) can set EINTR.
- Update STANDARDS to POSIX.1-2008.
- Note that setting errno is an extension to the spec.

Discussed with and revised by jmc@, deraadt@, millert@, and schwarze@.
With a history lesson from jsg@.

Thread: https://marc.info/?l=openbsd-tech&m=162718445809428&w=2

"the changes read fine to me" jmc@, ok millert@ schwarze@