* This updates stage names to use "spack-stage-" as a prefix.
This avoids removing non-Spack directories in "spack clean" as
c141e99 did (in this case so long as they don't contain the
prefix "spack-stage-"), and also addresses a follow-up issue
where Spack stage directories were not removed.
* Spack now does more-stringent checking of expected permissions for
staging directories. For a given stage root that includes a user
component, all directories before the user component that are
created by Spack are expected to match the permissions of their
parent; the user component and all deeper directories are expected
to be accessible to the user (read/write/execute).
1ef71376f2