build-sys: subtract list of exported symbols from "check-api-unused" list
bus: use ENXIO as error condition when trying to exit a container when we are not in any

So far we used ENXIO whenever the user tried to read something from a
function that didn't carry the right type, and hence use the same error
here to indicate that what the user expected and what was in the message
didn't match up.
bus: actually export sd_utf8_is_valid() and sd_ascii_is_valid()
bus: we really need to get rid of the :no-sender hack

This is where a hack from PID 1 spilled into the client side. We need
to get rid of this as soon as PID 1 is converted to libsystemd-bus.