Supervisor shutdown reason when reaching max restarts

Hi,

this patch changes the behaviour of supervisors to exit with a more specific reason when exiting due to a maximum restart limit hit. This is especially useful (or even necessary) to distinguish between normal and erroneous process terminations when monitoring a supervisor from another process.

In the above case a supervisor would now exit with {shutdown, {reached_max_restart_intensity, Child}} where Child is whatever is available to describe the child, either a child id or in case of a simple_one_for_one supervisor the offending child's process id. The patch should not affect the OTP restart behaviour (also for cascaded supervisors) since a subclass of 'normal' exit reasons is used.

I'm aware that there is some potential backward incompatibility for people that do not expect {shutdown, Reason} when monitoring a supervisor. However, the feature of exiting normally with {shutdown, Reason} has been around for quite a while now and I think this could be a sensible place to use it. Let me know what you think.

The patch does include tests and updated documentation.

          git fetch https://github.com/schlagert/otp.git supervisor_shutdown_reason

          https://github.com/schlagert/otp/compare/erlang:master...supervisor_shutdown_reason
          https://github.com/schlagert/otp/compare/erlang:master...supervisor_shutdown_reason.patch

Regards
Tobias
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