SA11-022 fix overflow on computation of memlimit
If a memlimit larger than 2GB is specified, the computation in the int type here will overflow before storing it in the field of type rlim_t. Converting to rlim_t before the computation.
Also, changing memlimit type to unsigned.
Change-Id: I58edce2bd007dce82045b17ef8f15c7c079009b1