Fix unecessary delays and timing jitter in the implementation of poll(): No delay if timeout is zero; If timeout is greater than zero, first check if events are already pending events before starting the delay.  From Johannes Hampel

Completes the implemenation of the core shared memory logic:  shmget(), shmctl(), shmat(), and shmdt().  This is still some unfinish platform-specific code that needs to be done before we can begin testing.

Add support for a per-process virtual page allocator.  This is a new member of the task_group_s structure.  The allocaor must be initialized when a new user process is started and uninitialize when the process group is finally destroyed.  It is used by shmat() and shmdt() to pick the virtual address onto which to map the shared physical memory.