More Perl malloc debugging magic from Ilya. Seems to work in

More Perl malloc debugging magic from Ilya.  Seems to work in
Linux, Solaris, AIX.  Had to do #ifdef OS2 for the <io.h> in
malloc.c, found in AIX since there is no such header.
In Tru64 miniperl fails an assert: "free()ed/realloc()ed-away
memory was overwritten?"
(In IRIX compiles but that doesn't prove much since in IRIX
Perl's malloc is simply not used.)

p4raw-id: //depot/perl@19831