Added --debug-assert-on-error variable which, if set, will cause safe_mutex to assert if it founds an error.

include/my_sys.h:
  Added my_assert_on_error
mysys/my_static.c:
  Added my_assert_on_error
mysys/thr_mutex.c:
  Assert when found wrong mutex usage if my_assert_on_error is set
sql/mysqld.cc:
  Added setting of my_assert_on_error