Improve the unsupported codec errors

Provide a description of the codec if possible
Give a more descriptive message if the codec is "undf"
That one means that the demuxer could not identify the codec
avcommon.h: move code around
libavutil: set verbosity based on VLC own verbosity
Simplify message when codec could not be found

People do not read that far and keep asking for support anyway.
std: Always use module name if access was not specified