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