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/api/api.go

@@ -1925,7 +1925,7 @@ func shouldRegenerateCertificate(cert tls.Certificate) error {
 	// On macOS, check for certificates issued on or after July 1st, 2019,
 	// with a longer validity time than 825 days.
 	cutoff := time.Date(2019, 7, 1, 0, 0, 0, 0, time.UTC)
-	if runtime.GOOS == "darwin" &&
+	if build.IsDarwin &&
 		leaf.NotBefore.After(cutoff) &&
 		leaf.NotAfter.Sub(leaf.NotBefore) > 825*24*time.Hour {
 		return errors.New("certificate incompatible with macOS 10.15 (Catalina)")

lib/api/api_test.go

@@ -19,7 +19,6 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
-	"runtime"
 	"strconv"
 	"strings"
 	"testing"
@@ -27,6 +26,7 @@ import (
 
 	"github.com/d4l3k/messagediff"
 	"github.com/syncthing/syncthing/lib/assets"
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/config"
 	connmocks "github.com/syncthing/syncthing/lib/connections/mocks"
 	discovermocks "github.com/syncthing/syncthing/lib/discover/mocks"
@@ -1255,7 +1255,7 @@ func TestShouldRegenerateCertificate(t *testing.T) {
 		t.Error("expected no error:", err)
 	}
 
-	if runtime.GOOS == "darwin" {
+	if build.IsDarwin {
 		// Certificates with too long an expiry time are not allowed on macOS
 		crt, err = tlsutil.NewCertificateInMemory("foo.example.com", 1000)
 		if err != nil {
@@ -1416,7 +1416,7 @@ func TestSanitizedHostname(t *testing.T) {
 // be prone to false negatives if things change in the future, but likely
 // not false positives.
 func runningInContainer() bool {
-	if runtime.GOOS != "linux" {
+	if !build.IsLinux {
 		return false
 	}