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/migrations.go

@@ -11,11 +11,11 @@ import (
 	"os"
 	"path"
 	"path/filepath"
-	"runtime"
 	"sort"
 	"strings"
 	"sync"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/upgrade"
 	"github.com/syncthing/syncthing/lib/util"
@@ -189,7 +189,7 @@ func migrateToConfigV24(cfg *Configuration) {
 
 func migrateToConfigV23(cfg *Configuration) {
 	permBits := fs.FileMode(0777)
-	if runtime.GOOS == "windows" {
+	if build.IsWindows {
 		// Windows has no umask so we must chose a safer set of bits to
 		// begin with.
 		permBits = 0700