Correctly set ipv4 and ipv6 enabled flags.

When an the flag is currently disabled in the system (e.g.
by an old config), a new config (ifup) has to explicitly
enable it and not keep unchanged (send a disable only).
Merge pull request #300 from mtomaschewski/reenable

ifreload: correctly set ipv4 and ipv6 enabled flags disabled before.