MEDIUM: cpu-topo: Add an optional directive for per-group affinity

MEDIUM: cpu-topo: Add an optional directive for per-group affinity

When using per-group affinity, add an optional new directive. It accepts
the values of "auto", where when multiple thread groups are created, the
available CPUs are split equally across the groups, and is the new
default, and "loose", where all groups are bound to all available CPUs,
this is the old default.