fixed multiple gzip/sendfile problems in css and js dispatchers FS#1571

- Avoid double compression when gzip_output is enabled
- Only compress when gzip_output is enabled
- Do not use x-sendfile for compressed content (content-encoding is not supported)
- Make sure the script terminates after using x-sendfile
- Moved gzip browser support check to init.php

darcs-hash:20090122215010-7ad00-765765d353ff78df5b8704086328c5c699bbe7e0.gz