Correct types of variables used to track amount of allocated SysV shared

Correct types of variables used to track amount of allocated SysV shared
memory from int to size_t. Implement a workaround for current ABI not
allowing to properly save size for and report more then 2Gb sized segment
of shared memory.

This makes it possible to use > 2 Gb shared memory segments on 64bit
architectures. Please note the new BUGS section in shmctl(2) and
UPDATING note for limitations of this temporal solution.

Reviewed by:	csjp
Tested by:	Nikolay Dzham <i levsha org ua>
MFC after:	2 weeks