If a SOCKS5 client insists on authentication, allow it to
negotiate a connection with Tor's SOCKS server successfully.
Any credentials the client provides are ignored.
This allows Tor to work with SOCKS5 clients that can only
support 'authenticated' connections.
Also add a bunch of basic unit tests for SOCKS4/4a/5 support
in buffers.c.
edge_connection_t: want_onehop if it must attach to a circuit with
only one hop (e.g. for the current tunnelled connections that use
begin_dir), and use_begindir if we mean to use a BEGIN_DIR relay
command to establish the stream rather than the normal BEGIN. Now
we can make anonymized begin_dir connections for (e.g.) more secure
hidden service posting and fetching.
svn:r12244
