dnvlist_take_string
actual_val = dnvlist_take_string(nvl, key, default_val);
actual_val = dnvlist_take_string(nvl, "123", default_val);
actual_val = dnvlist_take_string(nvl, "otherkey", default_val);
char *dnvlist_take_string(nvlist_t *nvl, const char *name, char *defval);