shell-completion: fix systemctl set/unset/import-environment

shell-completion: fix systemctl set/unset/import-environment

unset-environment is completed with variable names in the environment block.
set-environment the same, but suffixed with "=".
import-environment is completed with variable names in the client environment.

(cherry picked from commit 341992081b6ece1adba270e239f96c9840884885)