* Use gettext in "scripts/calcurse-upgrade".
* Add "scripts/calcurse-upgrade" to "po/POTFILES.in".
* Rename "scripts/calcurse-upgrade" to "scripts/calcurse-upgrade.sh" to
make sure xgettext(1) detects the correct input file format.
Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>