When using DEBUG_LEAKING_SCALARS_FORK_DUMP it's possible to cause

When using DEBUG_LEAKING_SCALARS_FORK_DUMP it's possible to cause
indefinite hangs when the debugging child holds open Perl_debug_log,
and it happens to be a file descriptor that is one end of a pipe,
with a process sitting at the other end waitng for EOF.
So close all descriptors in the child *including* Perl_debug_log,
and pass it back in over the control socket if it's needed.

p4raw-id: //depot/perl@24984