sb-posix vs Snow Leopard

Trying to build SBCL on Snow Leopard today, I ran into some errors in  
contrib/sb-posix.

Specifically, test.mkdir.3 and test.rmdir.3 fail because mkdir/rmdir  
fail because Snow Leopard throws EISDIR when you try to mkdir/rmdir /.  
I'm guessing that this may be because / is a mount point. Changing  
these tests to use /usr instead works (on Snow Leopard, at least),  
but /usr is itself a candidate for being a separate mount point.

Next, stat-mode.6 fails. I thought this was because sb-posix had the  
wrong value for S_IFSOCK, but that does no appear to be the case...  
instead, it appears that sb-posix:stat or sb-posix:stat-mode picks up  
the wrong value. This could (possibly) be an alignment issue.

Using a small C program I get (for the directory test-lab in contrib/ 
sb-posix):

mode = 04075 (note: octal)

using (format t "~o~%" (sb-posix:stat-mode (sb-posix:stat #p"sb-posix/ 
test-lab/"))), I get

146034

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