Use safe_unpackstr_xmalloc() instead of unpackstr_ptr to ensure all
strings are escaped correctly in SlurmDBD.

This requires the introduction of the _free_local_*() functions to free
the structure members, whereas they were previously just pointers into
the single managed buffer which was cleaned up elsewhere.

Remove the unsafe unpackstr_ptr macro definition as well while here.

CVE-2019-12838.