Utilities::getTempDir: Make sure that the temporary directory is owned by the current user.

This fixes a potential situation where another user creates the
temporary before simpleSAMLphp attempts to create it. Anther user
creating the directory could make simpleSAMLphp overwrite arbitrary
files on the system.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2081 44740490-163a-0410-bde0-09ae8108e29a