rename IANA language file

it's not a CSV but a plain text file
symlink for testing purpose
make bash script to retrieve check utitlity files more robust
IANA language file contains ambiguous entries, make them unique
CheckIANAEncoding

* specfile is a text file, not CSV
* bufio.ReadString includes the delimiter, get rid of it
* normalize input by lower-casing
* add tests for CheckIANAEncoding