Disable update checking per default, re-enable with --enable-update-check

Disable update checking per default, re-enable with --enable-update-check
Add OpenPGP code, still unused as there is no download of binaries at the moment

We will need to define (and write) the files (and their location) we will use in the update system:

    * We will use a file per arch, which will be signed
    * ALL VideoLAN gpg public keys will be stored on http://download.videolan.org/pub/keys/XXXXXXXXXXXXXXXX.asc where XXXXXXXXXXXXXXXX is the long id of the key
    * Every downloadable file will be signed with gpg --sign --detach --armor