Add a new virtual for OpenBSD service management: openbsdrcctl

It uses the rcctl(8) utility just introduced in OpenBSD (will be part of
OpenBSD 5.7) and implements all methods (enable, disable, ...).
Fix for #15172.
Implement disabled() and enabled().
Merge pull request #15172 from ajacoutot/develop

Add a new virtual for OpenBSD service management: openbsdrcctl