Ignore pid 0 in procapi on macosx. #3516

The full list of pids returned by sysctl() on MacOSX include pid 0,
which is not a real userland process. It's named kernel_task and
represents the kernel. Calls like task_for_pid() fail on it, leading to
worrying messages in the procd log. Ignore it in
ProcAPI::buildPidList().