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/config.c

@@ -39,6 +39,9 @@ Config *config_create(char *version, char *send_directory,
   config->max_size = 0;
   config->min_size = 0;
   config->use_incremental = false;
+  config->use_tls = false;
+  config->tls_cert = NULL;
+  config->tls_key = NULL;
   return config;
 }
 
@@ -74,6 +77,8 @@ void config_delete(Config *config) {
   for (int i = 0; i < config->include_count; i++)
     free(config->include_patterns[i]);
   free(config->include_patterns);
+  free(config->tls_cert);
+  free(config->tls_key);
   free(config);
 }