[9.20] fix: usr: Properly calculate the amount of system memory
requested to merge backport-4799-cid-498034-overflowed-return-value-integer_overflow-9.20 into bind-9.20
On 32 bit machines isc_meminfo_totalphys could return an incorrect value.
Closes #4799 (closed)
Backport of MR !9132 (merged)