Add generic daemon --opt and use it for some devmapper options

This adds a generic --opt used when starting the daemon which can be used for generic backend switches.
To prove this it also implements setting some devicemapper options:
    devmapper.basesize
    devmapper.loopbackdatasize
    devmapper.loopbackmetadatasize

Furthermore is makes failure during the initialization of a storage backend fatal, except for
when the backend is not supported. This is generally what you want as if you e.g. mistype
an option name you want the parse error reported, not have it silently use to the vfs backend.