Set CLOEXEC flag on the descriptors The received systemd descriptors don't have `O_CLOEXEC` set because they are received over `exec`. Thus if the process executes a child the child gets polluted with the descriptors. To prevent this, we set `O_CLOEXEC` during initialization. However this also required restructuring of the code because `libsystemd` doesn't provide temporary access to the descriptors - only permanent one. Thus we have to "validate" the descriptors eagerly. We still store the invalid ones as errors to make sure the errors get reported accurately.