Use MHD_create_response_from_fd for files

Now the library makes use of MHD_create_response_from_fd for files.
This improves the performances because allow to use sendfile on the
socket.
Use AC_SYS_LARGEFILE in configure.ac

This allow to avoid problems when working with FDs on 64bit systems
Avoid memory leak

Avoid a memory leak due to a typo (this is embarassing) in response
completion.