Use the SIO monad in the GF shell

+ The restrictions on arbitrary IO when GF is running in restricted mode is now
  enforced in the types.
+ This hopefully also solves an intermittent problem when accessing the GF
  shell through the web API provided by gf -server. This was visible in the
  Simple Translation Tool and probably caused by some low-level bug in the
  GHC IO libraries.

gf.cabal

@@ -112,7 +112,8 @@ executable gf
                  parallel
   ghc-options: -threaded
   if flag(server)
-    build-depends: httpd-shed, network, silently, utf8-string, json, cgi
+    build-depends: httpd-shed, network, utf8-string, json, cgi
+                   -- ,silently
     cpp-options: -DSERVER_MODE
     other-modules: GFServer
     hs-source-dirs: src/server src/server/transfer src/example-based