NA SM: use fallback if login information cannot be accessed
NA SM: use fallback if login information cannot be accessed

If login access fails on a system, use "unknown" as a placeholder string
to construct the /tmp paths to fall back gracefully.

Co-authored by: Phil Carns <carns@mcs.anl.gov>

mercury-hpc/mercury,mercury-hpc/mercury,mercury-hpc/mercury,mercury-hpc/mercury