PORTAGE_XATTR_EXCLUDE: add common user.* attributes (bug 640290)

Common user.* attributes should be safe to exclude, and they
are not supported on tmpfs, except for user.pax.* attributes
that are supported with the patch from bug 470644.

See: https://www.freedesktop.org/wiki/CommonExtendedAttributes/
Bug: https://bugs.gentoo.org/612612
Bug: https://bugs.gentoo.org/640290
1 file changed