Changes

make debootstrap work on filesystems mounted nodev

MR !109 added support for bind mounting devices when device creation
fails to facilitate running debootstrap inside unprivileged user
namespaces. If running debootstrap as root on a target filesystem
mounted nodev (such as /tmp since systemd turned it into a tmpfs by
default), creation of devices works, but the nodev option denies opening
such devices.

The check_sane_mount function is changed to not reject such mounts on
the grounds that a created null device is not writable. Rather, it
resumes trying the bind mount approach. Likewise, setup_devices now
verifies whether the created device nodes are usable and when they are
not (due to nodev), it bind mounts devices over devices thus making it
work on a nodev filesystem while still producing the same output as
before.

Closes: #1073169