elements: Move from a global is_elements to a global chainparams

We now have a pointer to chainparams, that fails valgrind if we do anything
chain-specific before setting it.

Suggested-by: Rusty Russell <@rustyrussell>

common/utils.h

@@ -13,6 +13,7 @@ extern secp256k1_context *secp256k1_ctx;
 /* FIXME: Instead of using this as a global, we might want to pass it as
  * context whenever we need it. The global var is just lazy... */
 extern bool is_elements;
+extern const struct chainparams *chainparams;
 
 /* Allocate and fill in a hex-encoded string of this data. */
 char *tal_hexstr(const tal_t *ctx, const void *data, size_t len);