Add a --debug-locks option.

Get rid of the "locked..." and "...unlocked!" messages by default, since
they're not usually interesting.  But add a new option to bring them back in
case we end up with trouble debugging the locking stuff.  (I don't really
100% trust it yet, although I haven't had a problem for a while now.)

redo.py

@@ -9,6 +9,7 @@ j,jobs=    maximum number of jobs to build at once
 d,debug    print dependency checks as they happen
 v,verbose  print commands as they are run
 shuffle    randomize the build order to find dependency bugs
+debug-locks  print messages about file locking (useful for debugging)
 """
 o = options.Options('redo', optspec)
 (opt, flags, extra) = o.parse(sys.argv[1:])
@@ -21,6 +22,8 @@ if opt.verbose:
     os.environ['REDO_VERBOSE'] = '1'
 if opt.shuffle:
     os.environ['REDO_SHUFFLE'] = '1'
+if opt.debug_locks:
+    os.environ['REDO_DEBUG_LOCKS'] = '1'
 
 is_root = not os.environ.get('REDO', '')