(#14333) Ensure module permissions are sane – 2.7.x backport.

Prior to this change, tarballs unpacked by the root user would be unpacked
with the permissions and the ownership information present in the tarball.
In the best case, this made some modules fail to function after installation
since the newly installed module was owned by a different user. In the worst
case, this could be seen as a fairly serious security vulnerability.

This change ensures that the module's permissions are always owner-writable,
world-readable, and that the owner is always the same as the module's parent
directory.