manager: run environment generators

Environment file generators are a lot like unit file generators, but not
exactly:

1. environment file generators are run for each manager instance, and their
   output is (or at least can be) individualized.

   The generators themselves are system-wide, the same for all users.

2. environment file generators are run sequentially, in priority order.

Thus, the lifetime of those files is tied to lifecycle of the manager
instance. Because generators are run sequentially, later generators can use or
modify the output of earlier generators.

Each generator is run with no arguments, and the whole state is stored in the
environment variables. The generator can echo a set of variable assignments to
standard output:

  VAR_A=something
  VAR_B=something else

This output is parsed, and the next and subsequent generators run with those
updated variables in the environment. After the last generator is done, the
environment that the manager itself exports is updated.

Each generator must return 0, otherwise the output is ignored.

The generators in */user-env-generator are for the user session managers,
including root, and the ones in */system-env-generator are for pid1.

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