uxc: add container management CLI tool

uxc: add container management CLI tool

As procd can now provide a fully fetured container runtime using ujail,
add a (for now) simple CLI tool to list, add, delete, start and stop
OCI-complaint container bundles and selecting whether they should be
launched on boot.
In future commits, this will be extended to provide state output, take
care of hooks, send signals and fetch remote container images in
accordance with the Open Container Initiative Runtime Specification.

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