runtime: clean up sigqueue.go
runtime: clean up sigqueue.go

Minor changes to make logic clearer.
Observed while working on the conversion.

LGTM=iant, dvyukov
R=dvyukov, iant
CC=golang-codereviews
https://codereview.appspot.com/140250043

firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old