Adds the following tests:
* if there are multiple shared mmaps for the same file, writes to
one mmap region should be visible in the other mmap regions.
* if you write to a file with write(), those changes should be
visible in shared mmaps for that file.
* if you write to a shared mmap, those changes should be visible
when you read() from the file.
* if you write to a private mmap, any changes you make should not
be visible when you read() the file.
MAP_FILE is not in POSIX, and is simply in most LibCs as a "default"
mode. Our own LibC defines it as 0, meaning "no flags". It is also not
defined in some OS's, such as Haiku. Let's be more portable and not use
the unnecessary flag.
