core/manager: move environment serialization out to basic/env-util.c

core/manager: move environment serialization out to basic/env-util.c

This protocol is generally useful, we might just as well reuse it for the
env. generators.

The implementation is changed a bit: instead of making a new strv and freeing
the old one, just mutate the original. This is much faster with larger arrays,
while in fact atomicity is preserved, since we only either insert the new
entry or not, without being in inconsistent state.

v2:
- fix confusion with return value