env-util: replace unsetenv_erase() by new getenv_steal_erase() helper

The new helper combines a bunch of steps every invocation of
unsetenv_erase() did so far: getenv() + strdup() + unsetenv_erase().
Let's unify this into one helper that is harder to use incorrectly. It's
in inspired by TAKE_PTR() in a way: get the env var out and invalidate
where it was before.

src/basic/env-util.h

@@ -70,3 +70,5 @@ int setenv_systemd_exec_pid(bool update_only);
 int getenv_path_list(const char *name, char ***ret_paths);
 
 int unsetenv_erase(const char *name);
+
+int getenv_steal_erase(const char *name, char **ret);