BecomeRootInUserNS re-exec podman in a new userNS. It returns whether podman was re-executed
into a new user namespace and the return code from the re-executed podman process.
If podman was re-executed the caller needs to propagate the error code returned by the child
process.
JoinUserAndMountNS re-exec podman in a new userNS and join the user and mount
namespace of the specified PID without looking up its parent. Useful to join directly
the conmon process.