units: conditionalize static device node logic on CAP_SYS_MODULES instead of CAP_MKNOD

npsawn containers generally have CAP_MKNOD, since this is required
to make PrviateDevices= work. Thus, it's not useful anymore to
conditionalize the kmod static device node units.

Use CAP_SYS_MODULES instead which is not available for nspawn
containers. However, the static device node logic is only done for being
able to autoload modules with it, and if we can't do that there's no
point in doing it.
units: conditionalize configfs and debugfs with CAP_SYS_RAWIO

We really don't want these in containers as they provide a too lowlevel
look on the system.

Conditionalize them with CAP_SYS_RAWIO since that's required to access
/proc/kcore, /dev/kmem and similar, which feel similar in style. Also,
npsawn containers lack that capability.
main: change check whether /etc is unpopulated to look for /etc/machine-id

Previously, we checked whether /etc was completely empty. This makes it
difficult though for container managers such as nspawn to install a
small number of files (such as /etc/timezone), and have the system
otherwise populate its own tree.

Hence, change this by looking for /etc/machine-id, which should be a
good sign whether /etc is populated or not.
nspawn: add new --volatile switch for booting containers in volatile (ephemeral) mode

Two modes are supported: --volatile=yes mounts only /usr into the
container, and a tmpfs as root directory. --volatile=state mounts the
full OS tree in, but overmounts /var with a tmpfs.

--volatile=yes hence boots with an unpopulated /etc and /var, starting
with pristine configuration and state.

--volatile=state hence boots with an unpopulated /var, only starting
with pristine state.