TLS transport: OpenSSL-based encrypted TCP

- New transport_tls.h/c: TLS server (server_create_tls, server_listen_tls)
  and client (client_connect_tls) using OpenSSL
- protocol.c: io_set_ssl() + SSL_read/SSL_write in send_n_data/receive_n_data
- transport_tcp.h: ssl/ssl_ctx fields added to Server/Client structs
- config.h/c: use_tls, tls_cert, tls_key fields
- client_cli.c: --tls, --cert, --key flags
- server.c: --tls, --cert, --key, -p flags with TLS support
- CMakeLists.txt: OpenSSL::SSL + OpenSSL::Crypto linkage
- shell.nix: openssl added to buildInputs

src/shared/transport_tcp.h

@@ -9,6 +9,7 @@ typedef struct Server {
   struct sockaddr_in address;
   unsigned int address_length;
   int file_descriptor;
+  void *ssl_ctx;
 } Server;
 
 typedef struct Client {
@@ -16,6 +17,8 @@ typedef struct Client {
   unsigned int address_length;
   int file_descriptor;
   pid_t ssh_child_pid;
+  void *ssl;
+  void *ssl_ctx;
 } Client;
 
 Server *server_create(int port);