1.3.1: relicense to MIT (clean-room CSS + drop copyleft-lib references)

Relicensed from AGPL-3.0 to MIT, effective 1.3.1 (<=1.3.0 remain AGPL). The CSS
content cipher and Stevenson title-key attack are attributed to their published
cryptanalysis (not libdvdcss); all libaacs/libbluray/libdvdread/libdvdnav name
references were dropped from comments while keeping the standard format/spec
descriptions. Also bumps to 1.3.1.

README.md

@@ -1,4 +1,4 @@
-[![License: AGPL-3.0](https://img.shields.io/badge/license-AGPL--3.0-blue)](LICENSE)
+[![License: MIT](https://img.shields.io/badge/license-MIT-blue)](LICENSE)
 
 # libfreemkv
 
@@ -190,4 +190,4 @@ Run `freemkv info disc:// --share` with the [freemkv CLI](https://github.com/fre
 
 ## License
 
-AGPL-3.0-only
+MIT