Fix bug #505406 - Make thunderbird work on Gentoo again

Changes made:
- Support thunderbird_tmp_t for /tmp created files and directories
- Support XDG types
- Make user content management optional (through access template)
Add description for generated tunables (thunderbird)
Add context for thunderbird (non-bin distro) as per bug #505406
Update file context definition for glsa-check
Mark python-exec/layman as portage_fetch_exec_t
Layman needs manage rights on portage cache (write to /var/cache/edb) as well as ebuild symlinks
Introduce shutdown_dontaudit_exec
Revert last change, not needed
tmpfiles on gentoo needs a few extra perms

currently tmpfiles.sh runs mknod which creates device_t
and is relabelled afterwards.
Merge Gentoo upstream