Log Message:
Update to 20190606. From the changelog:

Previously, an IPv6 split tunnel with a /128 request would result in a
default tunnel, rather than a specific route. Correctly set the default
route if we request a /0.

If `which ip` returns something on a non-Linux OS, it's an unrelated
tool that won't work for routing configuration; don't try to set IPROUTE
on another OS. This should fix the macOS issue discovered at:
<https://github.com/dlenski/openconnect/issues/132#issuecomment-470475009>