change hard coding of AF_UNSPEC to user-defined address-families per repository

Some environments that return AAAA records for their IMAP servers can pose
problems for clients that do not have end-to-end IPv6 connectivity for a number
of reasons (e.g. policy, lack of full routing, security, etc..)

Even with a fallback mechanism in place, you can still arrive at IMAP
implementations that could prevent authentication from unknown IPv6 space. This
in itself is not enough to fallback to IPv4 since there is an actual connection
on that socket.

This change is for introducing a user-defined value:

  [Repository imap-remote]
  ipv6 = no

to create a preference per repository on which AF to connect to the remote
server on

ipv6 = yes (AF_INET6)
ipv6 = no (AF_INET)
unspecified = default

Signed-off-by: Ebben Aries <e@dscp.org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/repository/IMAP.py

@@ -194,6 +194,9 @@ class IMAPRepository(BaseRepository):
 
         return self.getconfint('remoteport', None)
 
+    def getipv6(self):
+        return self.getconfboolean('ipv6', None)
+
     def getssl(self):
         return self.getconfboolean('ssl', 1)