[th/copying-update-license-text] COPYING: update GPL license text
This is the COPYING file from NetworkManager ([1]). It's basically the same. The difference are some only punctuation, white space and it refers to "Lesser" GPL (LGPL-2.1) instead of "Library" GPL (LGPL-2.0).
I think it's preferable to have the exact same license text in libnma as in NetworkManager.