[C] Add thread affinity support for the C media driver.

[mikeb01]

• Put thread affinity configuration on the driver context and add configuration variables.
• Manage configuration of thread affinity from aeronmd.c
• Add script to show a process and threads CPU affinity (linux only).

$ ./aeron-samples/scripts/show_thread_affinity.sh 
PID: 15596 (aeronmd)
pid 15596's current affinity list: 2 (aeronmd)
pid 15597's current affinity list: 3 (sender)
pid 15598's current affinity list: 5 (receiver)

[tmontgomery]

I like this better. I'm good with this if it passes CI.