This PR is the second attempt to translate builtin.
Server-sent translation files can be added to `builtin/locale/`, whereas client-side translations depend on gettext.
Otherwise, this updates all .po files for all line numbers that changed
since the last time updatepo.sh was run. We still add the filename, but
this information shouldn't change too often.
xgettext beginning with 0.18.4 supports this feature.
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.
