union checkout: Fix symlink handling for xattrs

Applying xattrs on a symlink during checkout failed since
it was setting the xattrs on the final filename, not the
temporary name.

This made the "checkout union 1" test in test-basic.sh
fail.

https://bugzilla.gnome.org/show_bug.cgi?id=741125
Add ot_lgetxattrat and ot_lsetxattrat utils

These are implementation of the missing corresponding syscalls that
are done with the /proc/self/fd mechanism described at:

https://mail.gnome.org/archives/ostree-list/2014-February/msg00017.html

https://bugzilla.gnome.org/show_bug.cgi?id=741125
Support for "bare-user" repo format

This format is pretty much the same as the "bare" format, except the
file ownership and xattrs is not stored in the actual filesystem object, but
rather on the side in a user xattr. This means two things:

1) An unprivileged user can store such a repo independent of the types
   of files in it or their xattrs. And you can later (as root)
   reconstruct the real filesystem tree with ownership. Although you
   can't do that using hardlink-sharing. This also means ostree
   fsck does a full verification.

2) Such a repository can be checked out with user-mode (checkout -U)
   as an unprivileged user using hardlinks for space sharing.

Additionally, symlinks are stored as regular files (with the content
being the symlink target) because user xattrs are not supported on
symlinks. We know at checkout time if the file is a symlink because
the original st_mode is stored in the xattr metadata.

https://bugzilla.gnome.org/show_bug.cgi?id=741125
Split out basic tests from test-basic.sh

This will let us reuse them with other repo types

https://bugzilla.gnome.org/show_bug.cgi?id=741125
Add test-basic-user.sh testing for bare-user repos

This just does whatever test-basic.sh does, but on a bare-user
repo.

This works standalone, but unfortunately it breaks in
gnome-desktop-testing-runner as /tmp doesn't support
xattrs.

https://bugzilla.gnome.org/show_bug.cgi?id=741125