tools/nolibc/types: add poll() and waitpid() flag definitions

- POLLIN etc were missing, so poll() could only be used with timeouts.
- WNOHANG was not defined and is convenient to check if a child is still
  running

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>