terminal-util: add pty_open_peer() helper

This opens a pty peer in one go, and uses the new race-free TIOCGPTPEER
ioctl() to do so – if it is available.

src/basic/terminal-util.h

@@ -144,3 +144,6 @@ int terminal_get_size_by_dsr(int input_fd, int output_fd, unsigned *ret_rows, un
 int terminal_fix_size(int input_fd, int output_fd);
 
 int terminal_is_pty_fd(int fd);
+
+int pty_open_peer_racefree(int fd, int mode);
+int pty_open_peer(int fd, int mode);