all OS functions should be "nothrow @trusted @nogc"

[Shachar Shemesh via Digitalmars-d]

The title really does says it all. I keep copying OS function 
declarations into my code, just so I can add those attributes to them. 
Otherwise I simply cannot call "signalfd" and "sigemptyset" (to name a 
couple from my most recent history) from @safe code.

I can try and set up a PR when I have the time. If anyone else wants to 
take an easy one before then, you're welcome to :-)

Shachar