runtime: remove sigmask type, use sigset instead

runtime: remove sigmask type, use sigset instead

The OS-independent sigmask type was not pulling its weight. Replace it
with the OS-dependent sigset type. This requires adding an OS-specific
sigaddset function, but permits removing the OS-specific sigmaskToSigset
function.

Change-Id: I43307b512b0264ec291baadaea902f05ce212305
Reviewed-on: https://go-review.googlesource.com/29950
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>