Add --check-release option.

Check release functionality will check the release directory
to ensure that none of the package files conflict. Packages
are allowed to have the same files as other files, but only
if the contents are identifical.
Use Python tarfile library, not tar utility.

In doing so, set the mtime, uid, gid, uname and gname to
constant values to avoid any leakage of data, and allow
simple regression checking of binaries.