Attempt to fix issue #196 - OS X not allowing fd access to memmap segment

Fixes an issue with memmaps on OS X. This arises because OS X does not allow file-like access to unbacked shared memory segments. However, the only need for the file access was to sidestep a problem in Linux where memmapping could appear to succeed but actually not. This does not seem to occur in OS X so at the moment it seems safe to remove that safety check based on platform.