Permit root-owned home directory

On certain types of filesystem (especially NFS appliances which support multiple operating systems), the user's home directory may report as being owned by root rather than the user, yet still permit the user to create and modify files normally (which will be owned by them).

Our users have home directories hosted on a NetApp storage appliance which uses mixed-mode ACLs but where the home directory is set up with NTFS ACLs at the top level. This means they have the expected effective permissions, but the ownership reports as root. This could also be the case if the filesystem were using NFS4 ACLs or similar.

This was originally raised downstream on launchpad as https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1152137 where it gained little attention, and we're still patching this locally (and we'd really like to stop doing that).

This would also resolve the use-case highlighted in !39 (closed), and wouldn't cause root-owned files if a user were to start the daemon as root, as was stated as the primary concern on that request.