MINOR: thread: report when thread-groups or nbthread results in less threads

MINOR: thread: report when thread-groups or nbthread results in less threads

Some setups where the number of threads is forced without any binding
(no cpu-map), are quite suspicious if they result in less threads than
available CPUs, and not even predictably bound, so we want to notify
the user that this might be an oversight.

Similarly, when thread-groups is forced and not nbthread (and no cpu-map),
and the final number of threads is lower than the hard-limit or the number
of CPUs we also indicate the impact and how to remedy it. This can happen
for example when starting on a machine with more than 64 CPUs and
thread-groups forced to 1, or on more than 128 CPUs and thread-groups
forced to 2 (e.g. when moving an older config to a new platform).

It is possible that some of these conditions might need to be readjusted
in the future to catch other traps or to relax certain commonly used,
valid cases, so for now it is preferable not to backport this patch.