deploy: Ensure that we can deploy using only /usr/lib/ostree-boot

rpm-ostree at least has the option to generate a tree with just that
instead of /boot, but while we were enumerating the latter, we'd still
return paths from /boot.

https://bugzilla.gnome.org/show_bug.cgi?id=740947
checkout: fchown symlink to proper uid/gid

When commiting a symlink we do store the uid/gid of the actual
symlink (i.e. not target). However, this was not restored
on non-user-mode checkout as it should.

This commit fixes that, and additionally it ensures xattrs
on symlinks are not set in user-mode checkout.

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