libcody: Make it buildable by C++11 to C++26

libcody: Make it buildable by C++11 to C++26

The following builds with -std=c++11 and c++14 and c++17 and c++20 and c++23
and c++26.

I see the u8 string literals are mixed e.g. with strerror, so in
-fexec-charset=IBM1047 there will still be garbage, so am not 100% sure if
the u8 literals everywhere are worth it either.

2025-11-21  Jakub Jelinek  <jakub@redhat.com>

	* cody.hh (S2C): For __cpp_char8_t >= 201811 use char8_t instead of
	char in argument type.
	(MessageBuffer::Space): Revert 2025-11-15 change.
	(MessageBuffer::Append): For __cpp_char8_t >= 201811 add overload
	with char8_t const * type of first argument.
	(Packet::Packet): Similarly for first argument.
	* client.cc (CommunicationError, Client::ProcessResponse,
	Client::Connect, ConnectResponse, PathnameResponse, OKResponse,
	IncludeTranslateResponse): Cast u8 string literals to (const char *)
	where needed.
	* server.cc (Server::ProcessRequests, ConnectRequest): Likewise.