This doesn't add any new command-line arguments, but it does mean that
a new command-line argument can be added for any variable, not just
those in `$variables{'global-defaults'}`, and its value will be copied
to `%globals`.

My main motivation for this commit is to make it possible to remove
the redundant variable declarations between
`$variables{'global-defaults'}` and
`$variables{'protocol-common-defaults'}`.

The code already treats `undef` and 0 the same, and `undef` omits them
from the recap which simplifies testing.

This is mostly to simplify tests, but it also improves readability.

The infrastructure changes in this commit also make it possible to
introduce a new `url` variable that defaults to `opt('server', $h)`
concatenated with `opt('script', $h)` so that we can start migrating
away from those user-unfriendly variables.

Now that the default changes depending on `usev4` and `usev6`, this is
no longer necessary.  Removing it simplifies the code a bit and makes
the behavior of unit tests match the overall behavior a bit better.

  * Delete values from `$config{$h}` and `$recap{$h}` when resetting
    values (as opposed to setting a falsy value).
  * Delete values from `$config{$h}` and `$recap{$h}` when they are no
    longer needed.

This is mostly done to improve the tests in `t/update_nics.pl`.

Rather than check whether `use`, `usev4`, or `usev6` is a non-disabled
value, check that `wantip`, `wantipv4`, or `wantipv6` is a defined
value.  This is preparation for removing the `status` variable in a
future commit.

When a legacy protocol implementation returns, move its `status` and
`ip` results to the new `status-ipv4` and `ipv4` (or `status-ipv6` and
`ipv6`) properties.

Also remove the now-unused `status` variable definition, and remove
`ip` from the recap.