validatefs: add new tool that enforces mount constraints

validatefs: add new tool that enforces mount constraints

This new tool looks for a three xattr on the root inode of a file system
that encode mount constraints of the file system. The tool is supposed
to be hooke into the mount logic and is supposed to protect against
misappropriating trusted file systems in unintended ways.

Consider the following scenario: we boot up on first boot and create a
tpm-locked pair of /var/ and /srv/ partitions via systemd-repart. An
attacker then offline modifies the partition table, exchanging the
metadata of the /var/ and /srv/ partition. So far we'd happily accept
that, honour the modified metadata and boot up. This could be used to
revert changes to /var/ or similar. And all that even though both
partitions are encrypted and locked to TPM!

With this new mechanism we can encode in the protected contents of the
file systems the ways it can be used: the partition type uuid, the
partition label and the intended mount point can be stored in xattrs,
and we can check them automatically on mount, and take action on
mismatch. (action would typically be immediate reboot).