nspawn: use three boolean fields from settings file when actually set

nspawn: use three boolean fields from settings file when actually set

Let's turn these three fields into tristates, so that we can distinguish
whether they are not configured at all from explicitly turned off.

Let#s then use this to ensure that we only copy the settings fields into
our execution environment if they are actually configured.

We already do this for some of the boolean settings, this adds it for
the missing ones.

The goal here is to ensure that an empty settings file used in
--settings=override mode (i.e. the default mode used in the
systemd-nspawn@.service unit) is truly a NOP.