Requires adding a sigfwd function for Solaris, as previously
signal2_unix.go was not built for Solaris.

Change-Id: Iea3ff0ddfa15af573813eb075bead532b324a3fc
Reviewed-on: https://go-review.googlesource.com/29550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>