core: Use at-relative functions for checking out tree copies too

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