If TracePoint#enable is passed a block, it previously started
the trace on all threads.  This changes it to trace only the
current thread by default.  To limit the scope of the change,
the current thread is only used by default if target and
target_line are both nil.  You can pass target_thread: nil
to enable tracing on all threads, to get the previous
default behavior.

Fixes [Bug #16889]