Get rid of the postrandom script. It was born in a time when the

Get rid of the postrandom script.  It was born in a time when the
random script ran before filesystems were mounted, which is no
longer the case.

In random_start(), immediately delete each file that is fed into
/dev/random, and recreate the default entropy file immediately
after reading and deleting it.  The logic used in random_stop()
to determine which file to write to should probably be factored
out and used here as well.