Auto merge of #1394 - Susurrus:sigevent_extra_traits, r=gnzlbg

Switch to manual trait impls for sigevent

`sigevent` on most platforms have padding or unused fields. Rather
than display those in the `Debug` impl by deriving it, manually implement
all `extra_traits` instead ignoring those fields.

I do worry that my `PartialEq` implementations for this for some platforms (like Linux) is not correct due to ignoring bytes that shouldn't be ignored because these structs don't have a proper union set up.

cc @asomers
Part of nix-rust/nix#1035