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.
Use EnumSet instead of ArrayList.

http://docs.oracle.com/javase/6/docs/api/java/util/EnumSet.html