Avoid MAP_POPULATE in TestMmap, OS X doesn't have it
Provide size for TestInterrupt

It seems it never saw the Read on OS X, after removing the
OpenDirectIO default.

It seems Linux sends the Read through to FUSE even if size was claimed
to be zero, while OS X does not.
Go 1.3 does not include syscall.Msync, so add it to syscallx

On Linux, munmap(2) is enough to flush the changed data to the file.
On OS X, an explicit msync(2) call is needed.

https://code.google.com/p/go/issues/detail?id=8245
TestMmap calls msync before expecting file writes

This used to fail on OS X.