tools/sched_ext: scx_qmap: Make debug output quieter by default

scx_qmap currently outputs verbose debug messages including cgroup operations
and CPU online/offline events by default, which can be noisy during normal
operation. While the existing -P option controls DSQ dumps and event
statistics, there's no way to suppress the other debug messages.

Split the debug output controls to make scx_qmap quieter by default. The -P
option continues to control DSQ dumps and event statistics
(print_dsqs_and_events), while a new -M option controls debug messages like
cgroup operations and CPU events (print_msgs). This allows users to run
scx_qmap with minimal output and selectively enable debug information as
needed.

Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>