recurse-dir: optionally, call callback when entering/leaving toplevel dir, too

recurse-dir: optionally, call callback when entering/leaving toplevel dir, too

So far recurse_dir() will call the callback whenever we enter a
directory, and then pass the struct dirent for that directory, and an fd
for the directory the dirent is part of (i.e. the parent of the
directory we call things for). For the top-level dir the function is
invoked for we will not call the callback however, because we have no
dirent for that, and not fd for the directory the top-level dir is part
of. Let's add a flag to call it anyway, and in that case pass a NULL
dirent and -1 as directory fd.

This is useful when we want to treat the top-level dir the same as any
dir further down.

This is done opt-in since the callback must be ablet to handle a NULL
dirent and a -1 directory fd.