linux/cpu: add idle/io_wait/nice/system/user percentages

[maximeborges]

As a first step to remove the deprecated heim dependency for vector, this PR adds support to retrieve the detailed CPU usage.

[GuillaumeGomez]

An important note: if it isn't implemented for all (or at least most) supported platforms, the feature will be rejected.

[maximeborges]

Looking a bit more into the code, I understand why, and started implementing it on *BSD also, thus why I switched the PR to draft :)

[GuillaumeGomez]

Ah nice. :)

One more thing: the src/common code is not supposed to contain cfg either. Ping me once ready for review.

[maximeborges]

For Vector I'd like to actually add getters for the raw values for the different times, would that be fine?
I'm not really sure if this is used by many people but that would be a blocking point to drop heim without a breaking change.

[GuillaumeGomez]

Hard to answer without code. However, if it's not unified across platforms, then no, unlikely to get merged.