Zalloc Potter and the Death-Free Hallows

Fix up them there ghosts!

• Stop orphaning their memory. They ARE PU_LEVEL, so they'll disappear eventually, but, like... it's not good memory management practice to just orphan them when you're literally never going to do anything with them ever again. Y'know?
• Make ghosts spawn properly on slopes.

No testwad needed, it should all be reasonably self-evident.