Move atoi() into atoi.py and add a new debug2() debug level.

atoi() was getting redundant, and unfortunately we can't easily load
helpers.py in some places where we'd want to, because it depends on vars.py.
So move it to its own module.

helpers.py

@@ -2,13 +2,6 @@ import sys, os, errno
 import vars
 
 
-def atoi(v):
-    try:
-        return int(v or 0)
-    except ValueError:
-        return 0
-
-
 def unlink(f):
     """Delete a file at path 'f' if it currently exists.
 
@@ -68,7 +61,10 @@ else:
 
 
 def debug(s):
-    if vars.DEBUG:
+    if vars.DEBUG >= 1:
+        _log('redo: %s%s' % (vars.DEPTH, s))
+def debug2(s):
+    if vars.DEBUG >= 2:
         _log('redo: %s%s' % (vars.DEPTH, s))