Use UUIDs to identify filesystems if kernel supports it.

Use UUIDs to identify filesystems if kernel supports it.

This introduces a new dependancy on libblkid.
If a filesystem being exported has a UUID that libblkid
can extract, then that is passed to the kernel for use
in identifying the filesystem in filehandles.
This means that 'fsid=' is no longer needed to work around the
problem of device numbers changing.
fsid= is still needed for fielsystems that have no device,
and can now be given  16byute uuid instead of just a 32bit one.