Add count_children/1 to supervisor.erl to determine the number of

children being managed without the memory impact of which_children/1

The function which_children/1 returns a list of the child processes
currently being supervised, but it has the penalty of creating a new
list thereby consuming more memory.  In low memory situations it is
often desirable to know which supervisor may have generated many
processes, but the act of discovering the culprit should not cause the
node to crash (or worse a different node if the kernel kills one
randomly).  The new function count_children/1 can give an indication
of which supervisor is taxing resources the most without adding to the
burden.  Rather than creating a new list, it walks the supervisor's
internal children structure using an accumulator function so that any
used memory can be incrementally collected yet the resulting count can
still be obtained.

The return result of count_children/1 is a property list of counts
containing:
  - {specs, Total_Num_Child_Specs}
  - {active, Num_Active_Child_Processes_Of_Supervisor_Or_Worker_Type}
  - {supervisors, Num_Supervisor_Type_Children_Including_Dead_Processes}
  - {workers, Num_Worker_Type_Children_Including_Dead_Processes}

This patch was made in response to mailing list discussions of the
problem diagnosing heavily taxed production systems.  I cannot find
the original request, but http://www.erlang.org/cgi-bin/ezmlm-cgi/4/35060
is my original post of the patch.

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