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_tls.h

@@ -0,0 +1,14 @@
+#ifndef TRANSPORT_TLS_H
+#define TRANSPORT_TLS_H
+
+#include "transport_tcp.h"
+#include <stdbool.h>
+
+bool tls_global_init(void);
+void tls_global_cleanup(void);
+
+bool server_create_tls(Server *server, const char *cert_path, const char *key_path);
+bool server_listen_tls(Server *server, void (*handler)(int file_descriptor));
+bool client_connect_tls(Client *client, char *host, int port, const char *cert_path, const char *key_path);
+
+#endif