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/protocol/bep_extensions.go

@@ -8,9 +8,9 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"runtime"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/sha256"
 )
@@ -286,15 +286,13 @@ func ModTimeEqual(a, b time.Time, modTimeWindow time.Duration) bool {
 }
 
 func PermsEqual(a, b uint32) bool {
-	switch runtime.GOOS {
-	case "windows":
+	if build.IsWindows {
 		// There is only writeable and read only, represented for user, group
 		// and other equally. We only compare against user.
 		return a&0600 == b&0600
-	default:
-		// All bits count
-		return a&0777 == b&0777
 	}
+	// All bits count
+	return a&0777 == b&0777
 }
 
 // BlocksEqual returns true when the two files have identical block lists.

lib/protocol/protocol_test.go

@@ -11,13 +11,13 @@ import (
 	"errors"
 	"io"
 	"os"
-	"runtime"
 	"sync"
 	"testing"
 	"testing/quick"
 	"time"
 
 	lz4 "github.com/pierrec/lz4/v4"
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/testutils"
 )
@@ -848,7 +848,7 @@ func TestIsEquivalent(t *testing.T) {
 		},
 	}
 
-	if runtime.GOOS == "windows" {
+	if build.IsWindows {
 		// On windows we only check the user writable bit of the permission
 		// set, so these are equivalent.
 		cases = append(cases, testCase{