MDEV-30453 Setting innodb_buffer_pool_filename to an empty string attempts to delete the data directory on shutdown

Let us make innodb_buffer_pool_filename a read-only variable
so that a malicious user cannot cause an important file to be
deleted on InnoDB shutdown. An attempt to delete a directory
will fail because it is not a regular file, but what if the
variable pointed to (say) ibdata1, ib_logfile0 or some *.ibd file?

It does not seem to make much sense for this parameter to be
configurable in the first place, but we will not change that in order
to avoid breaking compatibility.