reviseRootDir: skip default values, add validation
reviseRootDir: skip default values, add validation

1. In case --root option is not provided, do nothing.

2. Instead of checking if root value is empty string, check it after
   filepath.Abs, and reject "/". Improve docstring while at it.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

kolyshkin/runc,kolyshkin/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc,kolyshkin/runc,opencontainers/runc,opencontainers/runc