littlecheck: pass close_fds to subprocess.Popen

Closes #6435.

close_fds=True is actually the default in Python 2.7 and 3.2, but not in
ancient (but still in production in Red Hat Enterprise Linux 6) Python
2.6. Enable it there as well.

build_tools/littlecheck.py

@@ -250,7 +250,12 @@ class TestRun(object):
         if self.config.verbose:
             print(self.subbed_command)
         proc = subprocess.Popen(
-            self.subbed_command, stdin=PIPE, stdout=PIPE, stderr=PIPE, shell=True
+            self.subbed_command,
+            stdin=PIPE,
+            stdout=PIPE,
+            stderr=PIPE,
+            shell=True,
+            close_fds=True,  # For Python 2.6 as shipped on RHEL 6
         )
         stdout, stderr = proc.communicate()
         outlines = [