config: clarify source mount
config: clarify source mount

- The 'source' of a bind mount can either be a file or a directory.

- The 'type' field is a dummy  in case of bind mounts, often left to
"none" (in that case it does not have to be something listed in
/proc/filesystems).

- 'Relative' paths can only be used for bind mounts, not for other
mounts. That's how runc manages this.

Replaces: https://github.com/opencontainers/runtime-spec/pull/967

Signed-off-by: Alban Crequy <5d61c11e6df8ed8ffd2a3627a86b97e825765970@kinvolk.io>

opencontainers/runtime-spec,hqhq/runtime-spec,opencontainers/specs,laijs/specs