Is the mutual exclusion of dco and iproute2 still necessary?

AFAICS the --enable-iproute2 and --enable-dco configure options were made mutually exclusive, because at the time iproute2 was not able to create ovpn interfaces. This appears to be possible now at least on newer distributions and when the ovpn module is loaded.

Therefore I suggest to remove the code that prevents using these two options together from configure.ac.