Gettext + misc fixes

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

Conflicts (from the introduction of fr translations):
	src/CMakeLists.txt

Add french translation.

added explanation why libintl must not belinked under some oses

* regression fix for windows build.

This unbreaks the Windows build which would otherwise have problems at
the jthread level.

gettext paths problem

Some misc usability patches

When not running in place, check if the assumed data path in
../share/APPNAME exists. If it doesn't exist, fall back to ../data
instead. This allows the program to run successfully from the checkout
directory even without the run-in-place flag active.