System.Posix.Files.fileAccess fails inside OS X sandbox

Hi,

similar to what has been reported in https://ghc.haskell.org/trac/ghc/ticket/8741, the same code fails when run inside OS X' sandboxing mechanism because `access(2)` sets errno to `EPERM` rather than `EACCES` when access is denied due to sandboxing rules rather than file modes.

As with the ticket referenced above, this breaks Cabal on OS X when used inside a sandbox (which is the case for every MacPorts port):
```
Configuring stm-2.4.2...
Flags chosen: base4=True
Dependency array -any: using array-0.5.0.0
Dependency base ==4.*: using base-4.7.0.1
Setup: /usr/bin/ar: permission denied
```

The top commit adds `ePERM` as another errno value to be handled gracefully. Since the same code exists in the ByteString variant of the interface, I've also changed it there (and ported the change from \#8741, which was also missing). I took the liberty of removing the remaining tabs in this file, because there still were some in the block I changed.

I'm not sure whether this is the right place to open this ticket (http://hackage.haskell.org/package/unix says it is, \#8741 in GHC Trac suggests it isn't), so please let me know if I should take this patch elsewhere.