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
5ecd5fa58b