MEDIUM: cpu-topo: Add a "cpu-affinity" option

MEDIUM: cpu-topo: Add a "cpu-affinity" option

Add a new global option, "cpu-affinity", which controls how threads are
bound.
It currently accepts three values, "per-core", which will bind one thread to
each hardware thread of a given core, and "per-group" which will use all
the available hardware threads of the thread group, and "auto", the
default, which will use "per-group", unless "threads-per-core 1" has
been specified in cpu_policy, in which case it will use per-core.