Rewrite managed proxy environment setup code

Now, the environment setup is entirely OS-independent, as well as less
hacky and brittle.

changes/bug5105

@@ -0,0 +1,11 @@
+  o Minor bugfixes:
+
+    - Ensure that variables set in Tor's environment cannot override
+      environment variables which Tor tries to pass to a managed
+      pluggable-transport proxy.  Previously, Tor would pass every
+      variable in its environment to managed proxies along with the
+      new ones, in such a way that on many operating systems, the
+      inherited environment variables would override those which Tor
+      tried to explicitly set.  Bugfix on 0.2.3.12-alpha for most
+      Unixoid systems; bugfix on 0.2.3.9-alpha for Windows.
+