cpu_usage/linux: count iowait toward idle time

[tactikauan]

The iowait time present in /proc/stat is a sub-category of idle, and therefore shouldn't count toward CPU usage. However, Waybar doesn't account for it when calculating the idle time, giving wrong results in some cases.

For example, when running the following command on an idle system, to create a completely I/O bound task pinned to the first CPU:

taskset 1 dd if=/dev/sda of=/dev/null bs=1MB

Waybar displays the first CPU at 100% usage, which is incorrect, since the CPU is idle most of the time waiting for I/O, while other tools such as htop display the same CPU fairly close to 0%.