main_unix: simplify shm alloc/cleanup

Adapted from commit: https://github.com/cebix/macemu/commit/407edc8bfd3c6abc5da36392f8e4f5fa86b59a66

Now we don't get dead shm lying around if SheepShaver explodes in some nasty
way.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>