gettimeofday.2: miscellaneous manpage cleanup

gettimeofday.2: miscellaneous manpage cleanup

Highlights:

- Tweak the one-liner description.
- Better variable names.
- Reorder DESCRIPTION to reflect the importance of each interface.
- Advise against using gettimeofday(2) for measuring elapsed time.
- Isolate discussion of the historical timezone parameter to its
  own paragraph at the end of the DESCRIPTION.
- Update ERRORS.  Mention the securelevel(7) EPERM for settimeofday(2).
- Expand SEE ALSO.
- Note settimeofday(2) in STANDARDS.  It is available on many systems.

Discussed with jmc@, millert@, and deraadt@.  Possibly discussed with
schwarze@, though I can't find the email.

Thread: https://marc.info/?t=162765632800002&r=1&w=2

"reads fine to me" jmc@, ok millert@