perl built with USE_ITHREADS can deadlock during fork() or backticks

perl built with USE_ITHREADS can deadlock during fork() or backticks
since it doesn't ensure threads other than the one calling fork()
aren't holding any locks; the fix is to use pthread_atfork() to
hold global locks

building perl with -Dusemymalloc exacerbates the problem since
Perl_malloc() holds a mutex, and perl's exec() calls New()

XXX the code in win32thread.h may be needed on platforms that have
no pthread_atfork()

p4raw-id: //depot/perl@11151