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/config/config_test.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/d4l3k/messagediff"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -445,7 +446,7 @@ func TestVersioningConfig(t *testing.T) {
 }
 
 func TestIssue1262(t *testing.T) {
-	if runtime.GOOS != "windows" {
+	if !build.IsWindows {
 		t.Skipf("path gets converted to absolute as part of the filesystem initialization on linux")
 	}
 
@@ -538,7 +539,7 @@ func TestFolderCheckPath(t *testing.T) {
 			path: "link",
 			err:  nil,
 		})
-	} else if runtime.GOOS != "windows" {
+	} else if !build.IsWindows {
 		t.Log("running without symlink check")
 		t.Fatal(err)
 	}
@@ -593,7 +594,7 @@ func TestNewSaveLoad(t *testing.T) {
 }
 
 func TestWindowsLineEndings(t *testing.T) {
-	if runtime.GOOS != "windows" {
+	if !build.IsWindows {
 		t.Skip("Windows specific")
 	}