Fix directory self-testing logic

When I removed version_supports_begindir, I accidentally removed the
mechanism we had been using to make a directory cache self-test its
directory port.  This caused bug 6815, which caused 6814 (both in
0.2.4.2-alpha).

To fix this bug, I'm replacing the "anonymized_connection" argument to
directory_initiate_command_* with an enumeration to say how indirectly
to connect to a directory server.  (I don't want to reinstate the
"version_supports_begindir" argument as "begindir_ok" or anything --
these functions already take too many arguments.)

For safety, I made sure that passing 0 and 1 for 'indirection' gives
the same result as you would have gotten before -- just in case I
missed any 0s or 1s.