doc/examples/configure-index: make "Dpkg::Max{Arg,ArgBytes} match reality

Git-Dch: ignore
Use sysconf(_SC_ARG_MAX) to find the size of Dpkg::MaxArgBytes

Instead of hardcoding Dpkg::MaxArgBytes find out about it using
the sysconf(_SC_ARG_MAX) call.
Fix incorrect comparison between signed/unsigned

Git-Dch: ignore