Fix error while allocating shared memory.

Fix error while allocating shared memory.

If num_init_children * max_pool is huge, pool_coninfo_size() failed to
calculate the size of required shared memory because the data type
used for the variable to store shared memory size was int and it
overflows for too large shared memory size. To fix this change
pool_coninfo_size() to size_t and use appropriate size_t for the
variable used within the function.

Also fix to use proper format qualifier in
pool_shared_memory_create().