- Default scope is versioned with spack and can be overridden by site
  or user config.

- Default scope provides sensible default concretization preferences
  for all of Spack.

- per-platform concretization scope can be added later (to force a
  particular MPI on, e.g., Cray systems)

The share/man path should be in prefix_inspections for MANPATH as
share/man is a common location for manual pages.

Fixes #1072.