Remove unused variables

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Remove unused write_column() function

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Initialize t to silence uninit warning

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Annotate unused variables

Signed-off-by: Thomas Graf <tgraf@suug.ch>
build: Enable -Wall

Signed-off-by: Thomas Graf <tgraf@suug.ch>
build: fail CI builds if a new warning is introduced

Signed-off-by: Thomas Graf <tgraf@suug.ch>