Turbostat: Explain why _GNU_SOURCE is required
Turbostat: remove unused show_* variables
Turbostat: remove 'units' variable, only used once
Turbostat: Remove has_invariant_tsc

We refuse to run if it's not true, so let's just assume it is
Turbostat: remove has_aperf

We refuse to run if it's not true, so let's just assume it is
Turbostat: remove unused variable has_epb
Turbostat: refactor cpu probing

Reorganization and rewriting of the cpu-probing code:
- Only support genuine_intel CPUs
- Remove variables that are always expected to be true
- Make supported pkg/core cstates more comprehensive
- Simplify rafl code (remove unused warning feature: intervals too long)
Turbostat: reorder checks: start with 'uid == 0 ?'