CLEANUP: thread: indicate when max-threads-per-group is ignored

CLEANUP: thread: indicate when max-threads-per-group is ignored

Since it's easy to get caught by some parameters being ignored, let's
detect when mtpg was explicitly set and report a notice if it is ignored
due to thread-groups being set. For this we need to avoid presetting
the value in the global section and only set it when entering function
thread_detect_count(), which is OK since the value cannot be used before.