MYMALLOC for Win32:

MYMALLOC for Win32:
1. Initialize malloc_mutex before it is used (all platforms!)
2. Adjust #ifdef muddle to allow MYMALLOC and win32_ to coexist
3. Tweak win32/config*.* to define MYMALLOC
4. Provide sbrk() in terms of VirtualAlloc().

Also fixup -MT (perl95) build to handle Perl_current_thread
via call to DLL (as though an extension).

p4raw-id: //depot/ansiperl@259