Add parsing of user defined sorting specification and some helpers.
Example usage:
sortdef = "key1,key2,key3"
do {
id = compare_parse_key_to_id(&comp, sortdef)
if (id < 0) return error;
compare_add_sort_id(&comp, id)
} while(id >= 0);
Signed-off-by: David Sterba <dsterba@suse.com>