kernel config: turn off XFS in favor of USER_NS

XFS doesnt support the feature, and it must be explicitly disabled.
the USER_NS allows us to use dalias' superchroot.c to build sabotage
without rootperms.
unshare(CLONE_NEWUSER|CLONE_NEWNS)...
preliminary super_chroot support

super_chroot.c uses a new kernel feature (CONFIG_USER_NS),
introduced in linux 3.8, to provide chroot functionality
without root privs.

it was sent to the musl maillist in early august by dalias:
http://openwall.com/lists/musl/2013/08/01/2

our version is slightly adapted to do error handling and
bindmount the tarballs dir into the chroot.