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/fs/fakefs_test.go

@@ -17,6 +17,8 @@ import (
 	"sort"
 	"testing"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/build"
 )
 
 func TestFakeFS(t *testing.T) {
@@ -563,7 +565,7 @@ func testFakeFSRenameInsensitive(t *testing.T, fs Filesystem) {
 	}
 
 	// not checking on darwin due to https://github.com/golang/go/issues/35222
-	if runtime.GOOS != "darwin" {
+	if !build.IsDarwin {
 		if err := fs.Rename("/foo/bar/BAZ", "/FOO/BAR/bAz"); err != nil {
 			t.Errorf("Could not perform in-place case-only directory rename: %s", err)
 		}
@@ -786,7 +788,7 @@ func testFakeFSSameFile(t *testing.T, fs Filesystem) {
 			t.Fatalf("Could not create %s: %s", filename, err)
 		} else {
 			fd.Close()
-			if runtime.GOOS == "windows" {
+			if build.IsWindows {
 				time.Sleep(1 * time.Millisecond)
 			}
 		}