irqtop: use standard 'always/never' arguments instead of 'enable/disable'

irqtop: use standard 'always/never' arguments instead of 'enable/disable'

Most other util-linux tools that accept the option argument "auto"
additionally accept "always" and "never", not "enable" / "disable".

So, make option --cpu-stat accept those standard words (besides the
deviant ones), and adjust the documentation to use the standard words.

Furthermore, make --cpu-stat accept also "on", "off", "yes", "no",
"1", and "0", to allow the user to use shorter words.  The per-CPU
stats can now be suppressed with a succinct: `irqtop -c0`.

CC: Zhenwei Pi <pizhenwei@bytedance.com>
Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>