Patches to fix x86_64 last CD being empty (#500910)

[Bill Nottingham]

Jesse Keating (jkeating at redhat.com) said: 
> These patches fix bug #500910.  Essentially I'm making splittree create the
> split directories on demand as needed, rather than having pungi do some math
> to estimate how many splits would be needed.  It's a lot of line changes but
> not really too bad.  I've tested this in a few systems, srpm splits too.

As long as they're tested, +1. Do we have disc sanity scripts (files in the
right place, etc.) that we can already run?

Bill