down replication corruption


dbug/dbug.c:
  fixed DBUG_ macros to not crash if my_thread_init() had not been called
include/my_sys.h:
  keep track of shutdown, main, and signal threads for safemalloc
mysys/my_static.h:
  keep track of thread in safemalloc blocks
mysys/my_thr_init.c:
  use my_malloc()/my_free() in my_thread_init()
mysys/safemalloc.c:
  keep track of threads
  do not fuss if main,signal, or shutdown thread still has allocated blocks 
   when we call TERMINATE() as some blocks are freed later and there is no
   easy way to make TERMINATE() wait for those threads
sql/mysqld.cc:
  keep track of signal, main, and shutdown threads