WL#3951 - MyISAM: Additional Error Logs for Data Corruption

When table corruption is detected, in addition to current error message
provide following information:
- list of threads (and queries) accessing a table;
- thread_id of a thread that detected corruption;
- source file name and line number where this corruption was detected;
- optional extra information (string).

mysql-test/r/myisam_crash_before_flush_keys.result:
  Adjusted a test case according to WL#3951.
mysql-test/t/myisam_crash_before_flush_keys.test:
  Adjusted a test case according to WL#3951.
storage/myisam/CMakeLists.txt:
  Added mi_extrafunc.h to myisam sources list.
storage/myisam/Makefile.am:
  Added mi_extrafunc.h to myisam headers list.
storage/myisam/ha_myisam.cc:
  Added _mi_report_crashed() function (reports additional information
  whenever table corruption is detected).
storage/myisam/mi_extrafunc.h:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/mi_locking.c:
  For every call to mi_lock_database(F_[RD|WR|EXTRA_]LCK) - add
  current thread to the list of threads accessing this table.
  
  For every call to mi_lock_database(F_UNLCK) - remove current
  thread from the list of threads accessing this table.
storage/myisam/mi_test1.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/mi_test2.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/mi_test3.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/myisam_ftdump.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/myisamchk.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/myisamdef.h:
  Extra elements are added to MI_INFO and MYISAM_SHARE structures.
  MI_INFO is extended with LIST element, that holds a pointer to
  THD object accessing a table. MYISAM_SHARE is extended with LIST
  (list of threads accessing a table).
  
  Whenever table is marked as crashed, call mi_report_crashed() macro
  to provide useful information.
storage/myisam/myisamlog.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/myisampack.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/rt_test.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisam/sp_test.c:
  All standalone programs must define their version of _mi_report_crashed()
  by including mi_extrafunc.h.
storage/myisammrg/ha_myisammrg.cc:
  For each unedrlying table initialize `in_use' variable.