all: Add build constants for runtime.GOOS comparisons (#8442)

all: Add package runtimeos for runtime.GOOS comparisons

I grew tired of hand written string comparisons. This adds generated
constants for the GOOS values, and predefined Is$OS constants that can
be iffed on. In a couple of places I rewrote trivial switch:es to if:s,
and added Illumos where we checked for Solaris (because they are
effectively the same, and if we're going to target one of them that
would be Illumos...).

lib/upgrade/upgrade_common.go

@@ -15,6 +15,8 @@ import (
 	"runtime"
 	"strconv"
 	"strings"
+
+	"github.com/syncthing/syncthing/lib/build"
 )
 
 type Release struct {
@@ -236,15 +238,13 @@ func releaseNames(tag string) []string {
 	// standard, containing both the architecture/OS and the tag name we
 	// expect. This protects against malformed release data potentially
 	// tricking us into doing a downgrade.
-	switch runtime.GOOS {
-	case "darwin":
+	if build.IsDarwin {
 		return []string{
 			fmt.Sprintf("syncthing-macos-%s-%s.", runtime.GOARCH, tag),
 			fmt.Sprintf("syncthing-macosx-%s-%s.", runtime.GOARCH, tag),
 		}
-	default:
-		return []string{
-			fmt.Sprintf("syncthing-%s-%s-%s.", runtime.GOOS, runtime.GOARCH, tag),
-		}
+	}
+	return []string{
+		fmt.Sprintf("syncthing-%s-%s-%s.", runtime.GOOS, runtime.GOARCH, tag),
 	}
 }

lib/upgrade/upgrade_test.go

@@ -14,6 +14,8 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+
+	"github.com/syncthing/syncthing/lib/build"
 )
 
 var versions = []struct {
@@ -120,7 +122,7 @@ func TestSelectedRelease(t *testing.T) {
 }
 
 func TestSelectedReleaseMacOS(t *testing.T) {
-	if runtime.GOOS != "darwin" {
+	if !build.IsDarwin {
 		t.Skip("macOS only")
 	}