runner/executor: Limit reading dmesg to chunks

There was no disk limit checks in reading kernel dmesg and that
could lead to writing really huge dumps longer than 400MB,
greatly exceeding disk limits used by CI and hardly useful for
developers. Make a dmesg dumping in chunks, size depending on
number of CPUs present, with a minimum of 64KB.
This could also allow to kick in disk limits checks if a driver
starts spilling messages into dmesg.

v2: correct size at last dump, also inform user about exceeding
  limits (Kamil)

Closes: drm/igt-gpu-tools#129


Cc: Petri Latvala <adrinael@adrinael.net>
Cc: Karol Krol <karol.krol@intel.com>
Cc: Ewelina Musial <ewelina.musial@intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reviewed-by: Ryszard Knop <rk@dragonic.eu>