Change data_make to type-safe calls

[zuckschwerdt]

A preview for the (scripted) change from the data_make() varargs convention to the new type-safe and compact data api.

This can later be shortened from e.g.

data_t *data = NULL;
data = data_str(data, "model", ...);
data = data_int(data, "id", ...);

to

data_t *data = data_new();
data_str(data, "model", ...);
data_int(data, "id", ...);

if we use introduce a fixed header for data_t lists.