Why the strange name: the prefix is necessary to follow our own advice that
environment generators should have numerical prefixes. I also put -d- in the
name because otherwise the name was very easy to mistake with
systemd.environment-generator. This additional letter clarifies that this
on special generator that supports environment.d files.
v2:
- add example files to EXTRA_DIST
v3:
- rework for the new scheme where nothing is written to disk
v4:
- use separate dirs for system and user env generators
