Implement proposal 351

This proposal adds new syntax to the SOCKS5 username/password extension scheme, so that requests with usernames starting with are now reserved.

For C tor, all we need to do is reject every username starting with unless it is exactly "0".

@dgoulet, what do you think? Please look very closely, as I have likely forgotten C.

requested review from @dgoulet

assigned to @nickm

changed milestone to %Tor: 0.4.9.x-freeze

mentioned in issue arti#1615 (closed)

merged manually

Merged manually because I added a unit tests. Cheers!

mentioned in merge request torspec!280 (merged)

mentioned in commit torspec@0e977caf