Based on contribution https://bugs.mysql.com/file.php?id=26642&bug_id=90144

 from
Facebook, this patch introduces a new dynamic global system variable
innodb_buffer_pool_in_core_file which is ON by default.
When ON, everything should work in a backward compatible way.
Setting it to OFF (for example by passing --skip-innodb-buffer-pool-in-core-file)
means that the user does not want pages allocated for buffer pool to be included in
the core file in the event of dumping core.
If this intention can not be satisfied (for example the OS does not support marking
pages as MADV_DONTDUMP) then an error will be emitted to the log, as soon as this is
determined, and a core_file will be set to OFF, so no core file will be created at all
(which seems to be safer given user's intention).

This patch also:
- introduces linux-version.inc MTR helper file, which makes it easier to
  skip MTR test on old version of Linux.
- refactors all_persisted_variables.test, so that it outputs and uses correct numbers of
  system variables, lowering the effort required to keep it in sync with reality

Reviewed-by: Annamalai Gurusami <annamalai.gurusami@oracle.com>