bitcoin: Make chainparams const

`cli` and `cli_args` were not `const` before since they are added to a
non-`const` array. Using `cast_const` we can keep them `const` without
unsafe cast.

Reported-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Christian Decker <decker.christian@gmail.com>

bitcoin/chainparams.h

@@ -11,8 +11,8 @@ struct chainparams {
 	const char *network_name;
 	const struct sha256_double genesis_blockhash;
 	const int rpc_port;
-	char *cli;
-	char *cli_args;
+	const char *cli;
+	const char *cli_args;
 	const u64 dust_limit;
 
 	/* Whether this is a test network or not */

daemon/bitcoind.c

@@ -30,8 +30,8 @@ static char **gather_args(struct bitcoind *bitcoind,
 	size_t n = 0;
 	char **args = tal_arr(ctx, char *, 3);
 
-	args[n++] = bitcoind->chainparams->cli;
-	args[n++] = bitcoind->chainparams->cli_args;
+	args[n++] = cast_const(char *, bitcoind->chainparams->cli);
+	args[n++] = cast_const(char *, bitcoind->chainparams->cli_args);
 
 	if (bitcoind->datadir) {
 		args[n++] = tal_fmt(args, "-datadir=%s", bitcoind->datadir);