Document unused parameters.

Document unused parameters.

In all cases where unused parameters have to exist either because some API
requires it or for more consistency, they now have been documented using
__attribute__((unused)) (which is automatically disabled in case GCC is not
used to prevent problems with compilers not supporting attributes). In a few
other cases, the parameters have been removed.

This allows the use of -Wunused-parameter (which is enabled by -Wextra as
well, when using -Wall).