hardlink: implement --exclude-subtree

hardlink: implement --exclude-subtree

Now, it is possible to exclude files by their names, but it does not
allow for ignoring entire subtrees of the scanned hierarchy. The new
option only applies to directory names and forces the file-tree-walk
to skip the directory and all of its subdirectories.

This is based on FTW_SKIP_SUBTREE, which was originally only available
in glibc (since 2004). Therefore, the code is #ifdef-ed to make it
portable to other libc versions.

Addresses: https://github.com/util-linux/util-linux/discussions/3244
Signed-off-by: Karel Zak <kzak@redhat.com>