util-lib/systemd-run: implement race-free PTY peer opening (#34953)

This makes use of the new TIOCGPTPEER pty ioctl() for directly opening a
PTY peer, without going via path names. This is nice because it closes a
race around allocating and opening the peer. And also has the nice
benefit that if we acquired an fd originating from some other
namespace/container, we can directly derive the peer fd from it, without
having to reenter the namespace again.

src/basic/terminal-util.h

@@ -154,3 +154,6 @@ 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);