Implement DIR* cloning on Windows

Implement DIR* cloning on Windows

There doesn't seem to be a mechanism to clone FileFind handles on
Windows.  Therefore this implementation just reads all remaining
entries into a cache buffer and closes the handle.  All further
readdir() requests will be fulfilled from the cache buffer, in both
the original and the new interpreter.

This fixes bug 75154 on Windows (all tests in t/op/threads-dirh.t pass).

This commit also changes the return value of win32_telldir() to -1
for directory handles that have been read until the end.  The previous
return value was (NULL - dirp->start), which technically is not
valid C code.

API change alert: Perl_dirp_dup() gets an additional CLONE_PARAMS
parameter in this change (like all the other Perl_*_dup() functions).