units(1): Refactor the manual page and update usage information

units(1): Refactor the manual page and update usage information

Changes to units.1:

- Change the description to a more descriptive "conversion calculator".
- Sort options.
- Split the description into sections to make it easier to navigate the
  manual page.
- Improve the description of various options.
- Document the default value of the output format.
- Use more mdoc macros for better readability.
- Document the behavior of the PATH environmental variable.
- Improve examples.
- Add sections: EXIT STATUS, DIAGNOSTICS, and HISTORY.
- Document that units(1) cannot convert negative values and it handles long
  unit lists poorly.
- Update the documentation of the -V flag to match the implementation.
  units(1) prints its version and the units data file instead of its
  version and usage information.

Changes to units.c:

- Update usage information.
- Sort longopts elements.

This commit does not attempts to change the current behavior of units(1).
What's left to do is probably defining a better versioning (at the moment
units(1) always reports "FreeBSD units" as its version) and changing the
behavior of the -V flag to only print version.

Reviewed by:	allanjude (earlier version), bcr
Approved by:	bcr (mentor)
Differential Revision:	https://reviews.freebsd.org/D18977