fix another ifconfig syntax difference between Linux and *BSDs

fix another ifconfig syntax difference between Linux and *BSDs

See https://gitlab.com/openconnect/vpnc-scripts/-/merge_requests/9#note_466328301 :

> Thanks.  I don't know how this one was missed; the [FreeBSD man
> page](https://www.freebsd.org/cgi/man.cgi?ifconfig) and [macOS man
> page](https://ss64.com/osx/ifconfig.html) clearly show that `delete` has
> to come _after_ the address.
>
> However, Linux's ifconfig [requires `del` to come
> before](https://linux.die.net/man/8/ifconfig), and _only_ works for
> removing IPv6 addresses… so this needs to be reworked a bit to not break
> on Linux.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>