Fixes for the pot generation script.

Include ui menu files that are in any subfolders of main/ (previously,
the script was missing AlienCentered/ and khaos/).

Also, fix the file references in the generated pot file. Previously, these
were written with the gettext code comment tag (i.e. line starting with #.).
Using the proper file reference tag instead (line starting with #:) allows
gettext tools (like msgmerge) to merge and replace these lines when someone
updates a po file from an updated pot file, instead of duplicating them.
Localized the yesno/multi/etc. values in the UI.

In the menus, localized the choices that appears in front of the options,
that is to say stuff like "Yes", "No", "High", "Low"…
Revert debug leftover in last commit.
Renamed our N_() macro.

Renamed our N_() macro because it prevents us from using gettext's
in the UI part.
More localised strings, less localised debug msgs.

Localized quite a few messages here and there, and removed the localisation
of messages intended to developpers (debug, warning, errors). The idea is
that it should make it easier for everybody to solve problems if we have
common error messages.
Localised the tutorial texts.
Localised the pk3 autodownload screen.

Strangely, I had to remove the parenthesis of the N_() macro
to make q3lcc happy. I don't think this can hurt.
Added French translation.

game/fr.po is complete and reviewed, while client/fr.po still misses
quite a few strings. In pratice this means that some of the strings printed
on the console are still not translated, while nearly everything that appears
on the all the UIs is translated.
Merge pull request #158 from gilllux/master

Localisation improvement and French translation