This will make the manual testing of the incremental upgrade (for
Tails 1.2.1) a lot easier.

Otherwise, even with empty i18nspector output, we triggered !0 exit code, and
an empty line in the script's output.

It'll allow easier integration within our CI thingies.

This change has the drawback that we're buffering output, and displaying it only
once all i18nspector processes have completed. I say it's acceptable, now that
their accumulated runtime is quite low. There might be a way to do this better,
with some trick to capture stdout without buffering it.

For some reason, this new version outputs a dozen of blank lines or so.

We currently have more than 800 PO files. The previous code ran i18nspector
(and thus forked a python3 interpreter) for each of those.

Let's now:

  * run as many i18nspector in parallel as the system has CPU cores
  * pass as much as 64 files to i18nspector at a time (on a system with 4 cores,
    we should ideally pass a bit more argumants at a time; on a system with
    8 cores, we're close to the optimal theoretical setting; on a system with 16
    cores, 64 at a time is a bit too much)

On my system, total runtime has dropped from 73s to 3.8s.

That bug was fixed almost a year ago, and the current instructions
makes it look like you always should build with all proxy caching
disabled.

Nowhere else are we using the $/# convention.

While we're at it, provide code that can be copy-pasted for the "add
sources/pinning" instructions, to make the setup even easier. We also
fix the sections so it's clearer what should be done for each
supported Debian release.

The versions of vagrant currently in Debian cannot be used to build
Tails.