I ran out of inodes on the root partition upgrading beta5 to RC1, resulting
in a hopelessly confused system that will have to be rebuilt.  Apparently I
had enough disk space for the installer to start, but not enough inodes.

Now, it seems to me that it's much easier to calculate the required inode
usage than the required disk space usage, so I think that this should be
checked for.

I think the installer needs to check how many new files will be installed,
and ensure that enough inodes are available.