Use path allowed API for testing multipath

[huitema]

Improve the implementation of picoquic_probe_new_path_ex to incorporate tests and return detailed error codes.

Add API picoquic_subscribe_new_path_allowed to either return immediately if picoquic_probe_new_path_ex is expected to succeed, or program the stack to execute a callback picoquic_callback_next_path_allowed when the next call to picoquic_probe_new_path_ex is expected to succeed.

Use these two API in picoquicdemo to provide an example.

This should address issue #1835