All the variants that sets the new signal mask in minit do the same
thing, so merge them. This requires an OS-specific sigdelset function;
the function already exists for linux, and is now added for other OS's.

Change-Id: Ie96f6f02e2cf09c43005085985a078bd9581f670
Reviewed-on: https://go-review.googlesource.com/29771
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>