Make sleep abort request working again for Curses UI

1) Rework the sleep abort request to set the skipsleep configuration
   setting that the sleep() code checks.
2) Only output 15 rather than 50 debug messages on abort...

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/ui/UIBase.py

@@ -48,7 +48,7 @@ class UIBase(object):
         """list of debugtypes we are supposed to log"""
         self.debugmessages = {}
         """debugmessages in a deque(v) per thread(k)"""
-        self.debugmsglen = 50
+        self.debugmsglen = 15
         self.threadaccounts = {}
         """dict linking active threads (k) to account names (v)"""
         self.acct_startimes = {}