nspawn: fallback on bind mount when mknod fails

Some systems abusively restrict mknod, even when the device node already
exists in /dev. This is unfortunate because it prevents systemd-nspawn
from creating the basic devices in /dev in the container.

This patch implements a workaround: when mknod fails, fallback on bind
mounts.

Additionally, /dev/console was created with a mknod with the same
major/minor as /dev/null before bind mounting a pts on it. This patch
removes the mknod and creates an empty regular file instead.

In order to test this patch, I used the following configuration, which I
think should replicate the system with the abusive restriction on mknod:

  # grep devices /proc/self/cgroup
  4:devices:/user.slice/restrict
  # cat /sys/fs/cgroup/devices/user.slice/restrict/devices.list
  c 1:9 r
  c 5:2 rw
  c 136:* rw
  # systemd-nspawn --register=false -D .

v2:
 - remove "bind", it is not needed since there is already MS_BIND
v3:
 - fix error management when calling touch()
 - fix lowercase in error message

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