avutil/opt: display a better default value for int/int64 options

Example:

% ./ffmpeg -h encoder=aac
  -aac_coder         <int>        E...A... Coding algorithm (from -1 to 3) (default twoloop)
     faac                         E...A... FAAC-inspired method
     anmr                         E...A... ANMR method
     twoloop                      E...A... Two loop searching method
     fast                         E...A... Constant quantizer
[...]
