src/server: refactoring to isolate dependencies on the cgi/fastcgi packages

* Introducing the module CGI, re-exporting a subset of the cgi package. It
  might complete replace the cgi package in the future.
* Introducing the module CGIUtils, containing functions from FastCGIUtils that
  have nothing to do with fastcgi.

Some low level hackery with unsafePerformIO and global variables was left
in FastCGIUtils, but it is actually not used, neither for gf -server nor
exec/pgf-fcgi.hs.

src/server/gf-server.cabal

@@ -21,10 +21,13 @@ flag c-runtime
   Default:     False
 
 Library
-  exposed-modules: PGFService FastCGIUtils ServeStaticFile RunHTTP Cache
-  other-modules: URLEncoding Fold
+  exposed-modules: PGFService FastCGIUtils CGIUtils ServeStaticFile RunHTTP Cache
+  other-modules: URLEncoding CGI Fold
   hs-source-dirs: . transfer
 
+  build-depends: fastcgi >= 3001.0.2.2
+       -- Install it in Ubuntu with: apt-get install libghc-fastcgi-dev
+
   build-depends:   base >=4.2 && <5,
                    time, time-compat, old-locale,
                    directory,
@@ -41,6 +44,8 @@ Library
                    bytestring,
                    pretty,
                    random
+
+  ghc-options: -fwarn-unused-imports
   if os(windows)
     ghc-options: -optl-mwindows
   else
@@ -65,7 +70,7 @@ executable pgf-http
 executable pgf-service
   main-is: pgf-fcgi.hs
   Hs-source-dirs: exec
-  ghc-options: -threaded
+  ghc-options: -threaded -fwarn-unused-imports
   if impl(ghc>=7.0)
     ghc-options: -rtsopts