This solves two issues at once:
* CMake would delete po files during ‘make clean’ because it thought
  they were autogenerated and not just managed
* the only gettext tools readily available in Windows are so old they
  don't support options like --package-name

The change also moves minetest.pot down one level, so we don't need to
special case ‘en’ anymore.

The downside is of course that you need some sane POSIX shell to update
the po files.

Get rid of the system-specific updatelocales.sh and introduce an
updatepo cmake rule. po files are also updated before creating the mo
files, and we now keep the .pot file (in the po/en directory). To
stabilize the po file creation, file contents are sorted by source
filename.

Update po files in the process.

Get rid of the system-specific updatelocales.sh and introduce an
updatepo cmake rule. po files are also updated before creating the mo
files, and we now keep the .pot file (in the po/en directory). To
stabilize the po file creation, file contents are sorted by source
filename.

Update po files in the process.