[pacman-dev] Parallelize integrity checks

The following patches add a new _alpm_for_each_cpu() internal API function, and use it to spread out the workload of integrity checking deltas and packages over the available CPUs. I didn't end up dividing the packages by size, because the logic gets uglier for what is probably marginal benefit. So each thread gets roughly the same number of packages to check. It currently uses sysconf(_SC_NPROCESSORS_ONLN) if available to count the number of CPUs. Support for detecting the number of CPUs on other platforms should probably be added by people who can test on other platforms, but the git and x264 source code are both good examples.