This reverts commit open-mpi/ompi@f7257a8310.
Ensure that we properly cleanup the session directory tree. Prior code had issues with symlinks, especially if the file that the link points to was already removed as we traverse the tree. Also found that the dirent checks for directory type weren't fully portable, and so fall back to the stat-based approach which is known to be portable.
Fix singularity singletons by detecting we are in a container and properly setting the pmix selection to pick the isolated component. Remove a stale restriction blocking use of the sm btl
8c14df2328