Fix straggling MS_WINDOWS issues; add a changes file

There was one MS_WINDOWS that remained because it wasn't on a macro
line; a few remaining uses (and the definition!) in configure.in;
and a now-nonsensical stanza of eventdns_tor.h that previously
defined 'WIN32' if it didn't exist.

changes/win32macro

@@ -0,0 +1,4 @@
+  o Code simplifications and refactoring:
+    - Use the _WIN32 macro throughout our code to detect Windows.
+      (Previously we had used the obsolete 'WIN32' and the
+      idiosyncratic 'MS_WINDOWS'.)