Don't ref the implicit child watchers unless explicitly asked for; they...

Don't ref the implicit child watchers unless explicitly asked for; they shouldn't keep the loop alive since the user didn't ask for them

gevent/os.py

@@ -165,7 +165,7 @@ if hasattr(os, 'fork'):
             # we're not watching it
             return _waitpid(pid, options)
 
-        def fork_and_watch(callback=None, loop=None, fork=fork):
+        def fork_and_watch(callback=None, loop=None, ref=False, fork=fork):
             """
             Fork a child process and start a child watcher for it in the parent process.
 

gevent/subprocess.py

@@ -935,7 +935,7 @@ class Popen(object):
                     # write to stderr -> hang.  http://bugs.python.org/issue1336
                     gc.disable()
                     try:
-                        self.pid = fork_and_watch(self._on_child, self._loop, fork)
+                        self.pid = fork_and_watch(self._on_child, self._loop, True, fork)
                     except:
                         if gc_was_enabled:
                             gc.enable()