[ Squashed into single commit --RR ]
This adds two new macros, `p_req_tal()` and `p_opt_tal()`. These support
callbacks that take a `struct command *` context. Example:
static bool json_tok_label_x(struct command *cmd,
const char *name,
const char *buffer,
const jsmntok_t *tok,
struct json_escaped **label)
The above is taken from the run-param unit test (near the bottom of the diff).
The return value is true on success, or false (and it calls command_fail itself).
We can pretty much remove all remaining usage of `json_tok_tok` in the codebase
with this type of callback.
