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/exec/pgf-fcgi.hs

@@ -3,7 +3,7 @@ import Control.Concurrent(forkIO)
 import Network.FastCGI(runFastCGI,runFastCGIConcurrent')
 
 import PGFService(cgiMain,newPGFCache,stderrToFile,logFile)
-import System.IO
+
 main = do stderrToFile logFile
           fcgiMain =<< newPGFCache