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
