(redo) updates-proxy: explicitly block connection looping back to the proxy IP

Explicitly block something like "curl http://127.0.0.1:8082" and
return error page in this case. This error page is used in Whonix to
detect if the proxy is torrified. If not blocked, it may happen that
empty response is returned instead of error. See linked ticket for
details.

This was previously done for 10.137.255.254, but since migration to
qrexec-based connection, 127.0.0.1 is used instead.

Fixes QubesOS/qubes-issues#1482

network/updates-blacklist

@@ -1 +1,2 @@
 10.137.255.254
+127.0.0.1