Unify whitespace in System/Posix/Files/ByteString
Unify accepted errno flags for access

The ByteString variant of the access function didn't accept the same
flags as the non-ByteString one, but it makes sense that the OS doesn't
care about which one is being used and returns all error codes for both
variants.
Accept EPERM as valid error code for access(2)

This is useful on OS X when its sandboxing mechanism is used, because
that will set errno = EPERM when a file can't be written due to
sandboxing (as opposed to setting it to EACCES when file permissions
deny writing).