lightningd: check rpc names collisions with builtin commands

[Lagrang3]

We crashed if the name collision happened to be a builtin command.

lightningd: FATAL SIGNAL 11 (version v26.06-241-gb35b848-modded)
0x5573cd532b2c send_backtrace
	common/daemon.c:38
0x5573cd532bb6 crashdump
	common/daemon.c:83
0x7fe1fd4ccdef ???
	./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
0x5573cd4d51a4 plugin_rpcmethod_add
	lightningd/plugin.c:1396
0x5573cd4d5268 plugin_rpcmethods_add
	lightningd/plugin.c:1423
0x5573cd4d57ef plugin_parse_getmanifest_response
	lightningd/plugin.c:1805
0x5573cd4d68db plugin_manifest_cb
	lightningd/plugin.c:1827
0x5573cd4d2316 plugin_response_handle
	lightningd/plugin.c:692
0x5573cd4d7443 plugin_read_json
	lightningd/plugin.c:781
0x5573cd56fd01 next_plan
	ccan/ccan/io/io.c:60
0x5573cd57018c do_plan
	ccan/ccan/io/io.c:422
0x5573cd570245 io_ready
	ccan/ccan/io/io.c:439
0x5573cd571be3 io_loop
	ccan/ccan/io/poll.c:471
0x5573cd4a5a99 io_loop_with_timers
	lightningd/io_loop_with_timers.c:22
0x5573cd4d61d1 plugins_init
	lightningd/plugin.c:2063
0x5573cd4aad6c main
	lightningd/lightningd.c:1269
0x7fe1fd4b6ca7 __libc_start_call_main
	../sysdeps/nptl/libc_start_call_main.h:58
0x7fe1fd4b6d64 __libc_start_main_impl
	../csu/libc-start.c:360
0x5573cd47a120 ???
	_start+0x20:0
0xffffffffffffffff ???
	???:0

Changelog-Fixed: lightningd: checks for rpc name collisions with builtin commands before registering plugin

[nGoline]

lightningd/jsonrpc.c:1347: the p == NULL branch you're now handling is also reached with no collision at all. command_add() has already inserted command->name into rpc->cmdmap when json_escape_unescape_len() fails, and this path tal_free()s the cmd_and_usage without strmap_del() and before tal_add_destructor2() is set, so the map keeps a freed key and a freed value.

That unescape rejects \uXXXX, which jsmn accepts, so any plugin with a non-ASCII character in an rpcmethod usage/description gets there (pyln avoids it only via ensure_ascii=False). Before this patch it crashed in plugin_rpcmethod_add; with the patch the node boots and the first help segfaults at lightningd/jsonrpc.c:432 in json_add_help_command on cmd->command->depr_start.

	cmd->usage = json_escape_unescape_len(cmd, usage, strlen(usage));
	if (!cmd->usage) {
		strmap_del(&rpc->cmdmap, command->name, NULL);
		tal_free(cmd);
		return false;
	}

Verified: without it, a plugin declaring usage "arg Á" kills the node on help; with it, the plugin is rejected, the name is absent from help, and the node stays up.

lightningd/plugin.c:1402 — on that same path the new message says a builtin owns
the name when nothing does. Worth having jsonrpc_command_add() report which failure it was.