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.c

@@ -36,6 +36,7 @@ Server *server_create(int port) {
   server->address.sin_addr.s_addr = INADDR_ANY;
   server->address.sin_port = htons(port);
   server->address_length = sizeof(server->address);
+  server->ssl_ctx = NULL;
 
   if (bind(server->file_descriptor, (struct sockaddr *)&server->address,
            server->address_length) < 0) {
@@ -104,6 +105,8 @@ Client *client_create() {
   client->address.sin_family = AF_INET;
   client->address_length = sizeof(client->address);
   client->ssh_child_pid = -1;
+  client->ssl = NULL;
+  client->ssl_ctx = NULL;
   return client;
 }