[update] rename linux sys to posix

While not yet tested on other posix systems, it should largely be
compatible.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>
[update] more robust initialization

Now the size of the global shared memory structures are stored and
checked on initialization to make sure that the memory being attached to
has a matching layout.

Signed-off-by: Shahbaz Youssefi <ShabbyX@gmail.com>