Fixed syncronization bug with file-backed memory.

[Ex-32]

The current code calls munmap() without first invoking msync() on the same region: as explained in msync(2), this is a synchronization bug:

msync() flushes changes made to the in-core copy of a file that was mapped into memory using mmap(2) back to the filesystem. Without use of this call, there is no guarantee that changes are written back before munmap(2) is called.

This PR breaks out arena unmapping into an inline static function that first calls msync() with the MS_SYNC flag which requests a page sync for the mapped region and blocks until synchronization is complete, then unmaps the pages with munmap().