Improvements to integer option parsing

This is a re-roll of the first two patches in my previous series which used to include "filter-objects" which is now a separate topic.

[PATCH 1/2] Correct test-parse-options to handle negative ints

The first one has changed only in that I've moved the additional test to a more logical place in the test file.

[PATCH 2/2] Move unsigned long option parsing out of pack-objects.c

I've made the following changes to the second commit:

- renamed this OPT_MAGNITUDE to try and convey something that is
both unsigned and might benefit from a 'scale' suffix. I'm expecting
more discussion on the name!

- fixed the enum ordering to put this close to OPT_INTEGER

- added documentation to api-parse-options.txt

- marginally improved the opterror message on failed parses

- noted the change in behavior for the error messages generated for
pack-objects' --max-pack-size and --window-memory in the commit message