Minors fixes on Windows.

* Don't import daemonize if os.name=='posix'.
* Allow the controller to overwrite old .pid files upon startup. This
  was needed because the Windows HPC job scheduler kills jobs in a way
  that the signal handlers are not being run.

IPython/kernel/clusterdir.py

@@ -414,7 +414,7 b' class ApplicationWithClusterDir(Application):'
             open_log_file = sys.stdout
         log.startLogging(open_log_file)
 
-    def write_pid_file(self):
+    def write_pid_file(self, overwrite=False):
         """Create a .pid file in the pid_dir with my pid.
 
         This must be called after pre_construct, which sets `self.pid_dir`.
@@ -423,9 +423,11 b' class ApplicationWithClusterDir(Application):'
         pid_file = os.path.join(self.pid_dir, self.name + '.pid')
         if os.path.isfile(pid_file):
             pid = self.get_pid_from_file()
-            raise PIDFileError(
-                'The pid file [%s] already exists. \nThis could mean that this '
-                'server is already running with [pid=%s].' % (pid_file, pid))
+            if not overwrite:
+                raise PIDFileError(
+                    'The pid file [%s] already exists. \nThis could mean that this '
+                    'server is already running with [pid=%s].' % (pid_file, pid)
+                )
         with open(pid_file, 'w') as f:
             self.log.info("Creating pid file: %s" % pid_file)
             f.write(repr(os.getpid())+'\n')
@@ -442,7 +444,8 b' class ApplicationWithClusterDir(Application):'
                 self.log.info("Removing pid file: %s" % pid_file)
                 os.remove(pid_file)
             except:
-                pass
+                self.log.warn("Error removing the pid file: %s" % pid_file)
+                raise
 
     def get_pid_from_file(self):
         """Get the pid from the pid file.

IPython/kernel/ipclusterapp.py

@@ -20,7 +20,8 b' import os'
 import signal
 import sys
 
-from twisted.scripts._twistd_unix import daemonize
+if os.name=='posix':
+    from twisted.scripts._twistd_unix import daemonize
 
 from IPython.core import release
 from IPython.external import argparse

IPython/kernel/ipcontrollerapp.py

@@ -246,7 +246,7 b' class IPControllerApp(ApplicationWithClusterDir):'
     def start_app(self):
         # Start the controller service and set things running
         self.main_service.startService()
-        self.write_pid_file()
+        self.write_pid_file(overwrite=True)
         reactor.addSystemEventTrigger('during','shutdown', self.remove_pid_file)
         reactor.run()