core-util: Fix permissions handling while creating directories

core-util: Fix permissions handling while creating directories

This makes updating of permissions on existing directories optional with
pa_make_secure_dir() and pa_make_secure_parent_dir(). This makes sure
that the recursive directory creation doesn't end up modifying existing
directories, and also fixes a problem where creating an auth cookie
(specifically ~/.esd_auth) would end up modifying permissions on ~.
Thanks to Frédéric Danis for reporting this.