liboconfig: Relicensed to MIT license.

Florian Forster relicensed the code in collectd's Git commit
633c3966f770e4d46651a2fe219a18d8a9907a9f.

<https://github.com/collectd/collectd/commit/633c3966f770e4d46651a2fe219a18d8a9907a9f>