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.go

@@ -15,13 +15,13 @@ import (
 	"net"
 	"net/url"
 	"os"
-	"runtime"
 	"sort"
 	"strconv"
 	"strings"
 
 	"github.com/pkg/errors"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/protocol"
 	"github.com/syncthing/syncthing/lib/util"
@@ -549,7 +549,7 @@ loop:
 }
 
 func cleanSymlinks(filesystem fs.Filesystem, dir string) {
-	if runtime.GOOS == "windows" {
+	if build.IsWindows {
 		// We don't do symlinks on Windows. Additionally, there may
 		// be things that look like symlinks that are not, which we
 		// should leave alone. Deduplicated files, for example.