Detect when running in a containerized environment

[jjhursey]

If mpirun finds itself running in a containerized environment it may want to adapt to that environment. This could mean adjusting the default settings for some MCA parameters such as plm_rsh_chdir (as in PR #7092), or adding additional consistency checks.

Some container runtimes (e.g., Singularity) have a "tell" that can be inspected to determine if mpirun is inside of a container. Others do not. Investigating a broadly applicable detection mechanism (at least to the degree that we wish to support) is one challenge.

The benefit of this type of mechanism is generally more consistent user experience in various containerization scenarios. This may be able to be implemented as a new orted/mca/schizo component.