uxc: behave more like a compliant OCI run-time

uxc: behave more like a compliant OCI run-time

Follow CLI syntax as described in OCI run-time spec[1].
In addition, allow 'create' call also without 'path' parameter to
re-create previously created containers, also after reboot.

Usual workflow:
uxc create debian /mnt/sda3/debian
uxc start debian
uxc kill debian 1
uxc create debian
uxc start debian
...

To create a container and have it automatically launched at boot:
uxc create debian /mnt/sda3/debian true

 [1]: https://github.com/opencontainers/runtime-spec/blob/master/runtime.md#operations

Signed-off-by: Daniel Golle <daniel@makrotopia.org>