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.

The po file should not be named c55 if we're in ∆. But since it is not
exposed at installation time, we don't actually need its name to be
based on the project name at all, so just call it minetest.po

The po file should not be named c55 if we're in ∆. But since it is not
exposed at installation time, we don't actually need its name to be
based on the project name at all, so just call it minetest.po