Merge branch 'jk/unused-parameters'

Merge branch 'jk/unused-parameters'

Make our codebase compilable with the -Werror=unused-parameter
option.

* jk/unused-parameters:
  CodingGuidelines: mention -Wunused-parameter and UNUSED
  config.mak.dev: enable -Wunused-parameter by default
  compat: mark unused parameters in win32/mingw functions
  compat: disable -Wunused-parameter in win32/headless.c
  compat: disable -Wunused-parameter in 3rd-party code
  t-reftable-readwrite: mark unused parameter in callback function
  gc: mark unused config parameter in virtual functions