Prune process tree by PID if specified

[snprajwal]

Currently, the --ps-tree and --files flags print the entire process tree, along with the associated information. These flags can optionally use a PID, and prune the tree to show just that process and its children. This allows for reduced noise when the process that needs to be inspected is already known. E.g. the PID of a known process can be retrieved with --ps-tree, and the finer details can be printed specifically for that process with checkpointctl inspect --files --sk --pid <PID>. A short form -p can also be provided.