Add debconf helpers: Ask the user before building on a 32 bit kernel

Add debconf helpers: Ask the user before building on a 32 bit kernel

 * Detect at install time the kernel the user is running

   * If a 64-bit kernel is detected skip any question and just build
     the ZFS kernel module.

   * If a 32-bit kernel is detected, then the user is warned (via debconf)
     about that. And the user is asked to stop the build (the default
     is to stop the build). If the user selects to stop the build then
     we exit with status 0 on postinst before building the module so
     the package gets as installed correctly on the dpkg database but
     the module is not built.

   * If we can't detect if the kernel is 32 or 64 bit we tell the user that
     we couldn't detect that and we tell him that he shouldn't continue
     building ZFS on a 32-bit kernel.
     We also ask him to stop the build and we proceed like in the previous
     case.

  * If the user asks by mistake "no" to build the kernel and later he changes
    his mind and wants to build it, he just needs to reconfigure the package:
    dpkg-reconfigure zfs-dkms

 * TODO: Add translations for the debhelper templates.