redo-log: prioritize the "foreground" process.

When running a parallel build, redo-log -f (which is auto-started by
redo) tries to traverse through the logs depth first, in the order
parent processes started subprocesses.  This works pretty well, but if
its dependencies are locked, a process might have to give up its
jobserver token while other stuff builds its dependencies.  After the
dependency finishes, the parent might not be able to get a token for
quite some time, and the logs will appear to stop.

To prevent this from happening, we can instantiate up to one "cheater"
token, only in the foreground process (the one locked by redo-log -f),
which will allow it to continue running, albeit a bit slowly (since it
only has one token out of possibly many).  When the process finishes,
we then destroy the fake token.  It gets a little complicated; see
explanation at the top of jwack.py.

redo-ifchange.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python2
-import sys, os
+import sys, os, traceback
 
 import vars_init
 vars_init.init(sys.argv[1:])
@@ -20,7 +20,8 @@ def should_build(t):
 rv = 202
 try:
     if vars_init.is_toplevel:
-        builder.start_stdin_log_reader(status=True, details=True)
+        builder.start_stdin_log_reader(status=True, details=True,
+            debug_locks=False, debug_pids=False)
     if vars.TARGET and not vars.UNLOCKED:
         me = os.path.join(vars.STARTDIR, 
                           os.path.join(vars.PWD, vars.TARGET))
@@ -29,6 +30,7 @@ try:
     else:
         f = me = None
         debug2('redo-ifchange: not adding depends.\n')
+    jwack.setup(1)
     try:
         targets = sys.argv[1:]
         if f:
@@ -41,7 +43,12 @@ try:
         try:
             state.rollback()
         finally:
-            jwack.force_return_tokens()
+            try:
+                jwack.force_return_tokens()
+            except Exception, e:
+                traceback.print_exc(100, sys.stderr)
+                err('unexpected error: %r\n' % e)
+                rv = 1
 except KeyboardInterrupt:
     if vars_init.is_toplevel:
         builder.await_log_reader()