On Linux, CPUs can come online or go offline while uv_cpu_info() is busy
gathering data. Change uv_cpu_info() in the following ways:
1. Learn online CPUs from /proc/stat
2. Get the model name from /proc/cpuinfo when it has a matching CPU,
or default to "unknown"
3. Get speed from /sys/devices/system/cpu/cpu%u/cpufreq/scaling_cur_freq
when it exists, or default to 0
Before this commit, libuv read the speed from /proc/cpuinfo but that
reports the base frequency, not the actual frequency. My system has
two cores running permanently at 3.6 GHz but libuv thought all 12 ran
at 2.2 GHz.
Fixes: https://github.com/libuv/libuv/issues/2351
Fixes: https://github.com/libuv/libuv/issues/3858
434eb4b0ac