fileutil: Add API to retrieve xattrs relative to a directory fd

This is the only safe way to get xattrs for a symlink relative to a
dirfd.
local-alloc: Define cleanup functions for GKeyFile and GList

See https://github.com/hughsie/libhif/pull/2
localalloc: add gs_free_list

Signed-off-by: Thomas Haller <thaller@redhat.com>
localalloc: add gs_free_slist

Signed-off-by: Thomas Haller <thaller@redhat.com>
fileutil: avoid using PATH_MAX

Use the dynamically allocated buffer returned by realpath instead of
using PATH_MAX which is not defined on platforms like Hurd.