Add hw.ncpuonline to count the number of online CPUs.

Add hw.ncpuonline to count the number of online CPUs.

The introduction of hw.smt means that logical CPUs can be disabled
after boot and prior to suspend/resume.  If hw.smt=0 (the default),
there needs to be a way to count the number of hardware threads
available on the system at any given time.

So, import HW_NCPUONLINE/hw.ncpuonline from NetBSD and document it.
hw.ncpu becomes equal to the number of CPUs given to sched_init_cpu()
during boot, while hw.ncpuonline is equal to the number of CPUs available
to the scheduler in the cpuset "sched_all_cpus". Set_SC_NPROCESSORS_ONLN
equal to this new sysctl and keep _SC_NPROCESSORS_CONF equal to hw.ncpu.

This is preferable to adding a new sysctl to count the number of
configured CPUs and keeping hw.ncpu equal to the number of online
CPUs because such a change would break software in the ecosystem
that relies on HW_NCPU/hw.ncpu to measure CPU usage and the like.
Such software in base includes top(1), systat(1), and snmpd(8),
and perhaps others.

We don't need additional locking to count the cardinality of a cpuset
in this case because the only interfaces that can modify said cardinality
are sysctl(2) and ioctl(2), both of which are under the KERNEL_LOCK.

Software using HW_NCPU/hw.ncpu to determine optimal parallism will need
to be updated to use HW_NCPUONLINE/hw.ncpuonline.  Until then, such software
may perform suboptimally.  However, most changes will be similar to the
change included here for libcxx's std::thread:hardware_concurrency():
using HW_NCPUONLINE in lieu of HW_NCPU should be sufficient for determining
optimal parallelism for most software if the change to _SC_NPROCESSORS_ONLN
is insufficient.

Prompted by deraadt. Discussed at length with kettenis, deraadt, and sthen.
Lots of patch tweaks from kettenis.

ok kettenis, "proceed" deraadt