MDEV-6756: map a linux pid (child pid) to a connection id shown in the output of SHOW PROCESSLIST

Added tid (thread ID) for system where it is present.

 ps -eL -o tid,pid,command

shows the thread on Linux

sql/sql_class.h

@@ -2630,6 +2630,7 @@ public:
   ulong      query_plan_fsort_passes; 
   pthread_t  real_id;                           /* For debugging */
   my_thread_id  thread_id;
+  pid_t      os_thread_id;
   uint	     tmp_table, global_disable_checkpoint;
   uint	     server_status,open_options;
   enum enum_thread_type system_thread;