When excluding nodes by country, exclude {??} and {A1} too

This is ticket 7706, reported by "bugcatcher."  The rationale here
is that if somebody says 'ExcludeNodes {tv}', then they probably
don't just want to block definitely Tuvaluan nodes: they also want
to block nodes that have unknown country, since for all they know
such nodes are also in Tuvalu.

This behavior is controlled by a new GeoIPExcludeUnknown autobool
option.  With the default (auto) setting, we exclude ?? and A1 if
any country is excluded.  If the option is 1, we add ?? and A1
unconditionally; if the option is 0, we never add them.

(Right now our geoip file doesn't actually seem to include A1: I'm
including it here in case it comes back.)

This feature only takes effect if you have a GeoIP file.  Otherwise
you'd be excluding every node.

doc/tor.1.txt

@@ -689,6 +689,14 @@ The following options are useful only for clients (that is, if
     node listed in ExcludeNodes is automatically considered to be part of this
     list too.  See also the caveats on the "ExitNodes" option below.
 
+**GeoIPExcludeUnknown** **0**|**1**|**auto**::
+    If this option is set to 'auto', then whenever any country code is set in
+    ExcludeNodes or ExcludeEntryNodes, all nodes with unknown country (?? and
+    possibly A1) are treated as excluded as well. If this option is set to
+    '1', then all unknown countries are treated as excluded in ExcludeNodes
+    and ExcludeEntryNodes.  This option has no effect when a GeoIP file isn't
+    configured or can't be found.  (Default: auto)
+
 **ExitNodes** __node__,__node__,__...__::
     A list of identity fingerprints, nicknames, country codes and address
     patterns of nodes to use as exit node---that is, a