Add parameter to enable cpu affinity

I saw in multiple forks/papers about userspace click that authors added thread affinity. We needed it too for our work, and I saw improvement using thread affinity with netmap (with the help of irq affinity too).

I propose to add a parameter once and for all. Adding "-a" in commandline pins threads 0 to core 0, threads 1 to core 1, ... The default remains "without affinity" as affinity can be very bad too (with a kernel overwhelmed by packets at 40Gbits in PCAP and LINUX modes for example).

I also have a patch for pinning IRQ affinity of a NIC to the core handling the corresponding FromDevice, but it's quite platform-specific as it reads the content of /proc to find which irq is tied to the device, and then writes the thread index in /proc/irq/%d/smp_affinity, so I'll pull-request it only if someone think it could go directly into click.