Issue #3864: Skip three test_signal tests on freebsd6 due to platform bug.

Two itimer tests and an interprocess signal test fail on FreeBSD 6 if
any test that starts a thread runs before test_signal.  Since FreeBSD7
does not show this behavior, the bug is most likely a platform bug,
so this patch just skips the failing tests on freebsd6.
diff --git a/Misc/NEWS b/Misc/NEWS
index b53beab..9f1cba1 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -294,6 +294,9 @@
 Tests
 -----
 
+- Issue #3864: Skip three test_signal tests on freebsd6 because they fail
+  if any thread was previously started, most likely due to a platform bug.
+
 - Issue #8348: Fix test ftp url in test_urllib2net.
 
 - Issue #8204: Fix test_ttk notebook test by forcing focus.