core: Use linkat() for hardlink checkouts too

Clean up how we deal with the uncompressed object cache; we now use
openat()/linkat() and such just like we do for the main objects/.

Use linkat() between the objects and the destination, if possible.

https://bugzilla.gnome.org/show_bug.cgi?id=707733
core: Make write_object() a bit more efficient

Do as many operations as we can using the original file descriptor
while we have it open, rather than writing, closing, then reopening.

This necessitated very explicitly special casing symbolic links,
mainly due to the lack of lsetxattrat().

https://bugzilla.gnome.org/show_bug.cgi?id=707733
core: Make ostree_set_xattrs() private

Nothing external uses it.  We keep ostree_get_xattrs_for_file() public
because it's convenient for external consumers to get xattrs in
exactly the format we desire.

https://bugzilla.gnome.org/show_bug.cgi?id=707733
core: Use at-relative functions for checking out tree copies too

For the cases where we can't hardlink, use at-relative walking of the
path where possible.  We still don't have lsetxattrat, so we also need
to deal with pathnames, but that is now only for symlinks.

Again, the advantages of this are a lot less malloc() of pathnames in
ostree, and much less time spent traversing paths inside the kernel.

https://bugzilla.gnome.org/show_bug.cgi?id=707733