MINOR: use DISGUISE() everywhere we deliberately want to ignore a result

MINOR: use DISGUISE() everywhere we deliberately want to ignore a result

It's more generic and versatile than the previous shut_your_big_mouth_gcc()
that was used to silence annoying warnings as it's not limited to ignoring
syscalls returns only. This allows us to get rid of the aforementioned
function and the shut_your_big_mouth_gcc_int variable, that started to
look ugly in multi-threaded environments.