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/protocol.h

@@ -10,6 +10,8 @@ enum NET_STATUS { STATUS_OK, STATUS_ERROR, STATUS_FINISHED, STATUS_NEXT, STATUS_
 
 void io_set_fds(int read_fd, int write_fd);
 void io_set_bwlimit(unsigned long long bytes_per_sec);
+typedef struct ssl_st SSL;
+void io_set_ssl(SSL *ssl);
 bool send_n_data(int file_descriptor, void *data, size_t data_size);
 bool receive_n_data(int file_descriptor, void *data, size_t data_size);