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.
Actually, overriding toString so that it returns chmodForm in PosixFilePermissions would allow to use the EnumSet directly for required permissions in error messages.