ftw/nftw is limited by stack size

[Jim Meyering]

ftw/nftw currently uses explicit recursion, and as such is limited
by the size of the user's stack.

For example, those functions may not be used to traverse a hierarchy
of depth 40000;  a simple traversal-no-op program blows the stack
and gets a segfault on my system:

  $ perl -e '$i=0; do {mkdir "z",0700; chdir "z"} until (++$i == 40000)')
  $ ./a.out z
  Segmentation fault
  [Exit 139 (SIGSEGV)]

What do you think about rewriting it to use space on the heap
rather than on the stack?

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