Rename COPYING into LICENSE

These days, it's quite common to get license information in
LICENSE file.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Clarify source code that LGPL version 2.1 is used

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
libtree is now under LGPLv2.1+

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
bst: fix bstree_replace() to update the thread links

Thanks to 'zjx20' for spotting.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>