pid1: rework environment block copy logic

pid1: rework environment block copy logic

This reworks the logic introduced in
a5cede8c24fddda9b73f142e09b18b49adde1b9c (#13693).

First of all, let's move this out of util.c, since only PID 1 really
needs this, and there's no real need to have it in util.c.

Then, fix freeing of the variable. It previously relied on
STATIC_DESTRUCTOR_REGISTER() which however relies on static_destruct()
to be called explicitly. Currently only the main-func.h macros do that,
and PID 1 does not. (It might be worth investigating whether to do that,
but it's not trivial.) Hence the freeing wasn't applied.

Finally, an OOM check was missing, add it in.