Ensure `DDD_GID` is at least 64 bits wide

a0cf9c01 increased MAX_PROCBITS_IN_GID from 9 to 24. As an unsigned long is only 32 bits wide on common 32 bit architectures, this left only 8 bits instead of the intended 40 bits (on 64 bit architectures) for objects.

This patch makes sure the DDD_GID type is at least 64 bits by using the std::uint_least64_t type. It should fix the build failure on, for example, i386 and PowerPC.