Fix for https://issues.infn.it/jira/browse/VOMS-529

Currently, APIs are overly strict in requiring 400 permissions on the
key file.

Details:
Extended functionality of checkPrivateKeyPermissions accepting as
argument a list of posix file permissions.
Added related unit test.
I don't get this loop.
If perms are in the set of accepted permissions, then checks terminates.
Otherwise, raise InvalidFilePermission error.