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/basicfs.go

@@ -11,11 +11,11 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"runtime"
 	"strings"
 	"time"
 
 	"github.com/shirou/gopsutil/v3/disk"
+	"github.com/syncthing/syncthing/lib/build"
 )
 
 var (
@@ -79,7 +79,7 @@ func newBasicFilesystem(root string, opts ...Option) *BasicFilesystem {
 
 	// Attempt to enable long filename support on Windows. We may still not
 	// have an absolute path here if the previous steps failed.
-	if runtime.GOOS == "windows" {
+	if build.IsWindows {
 		root = longFilenameSupport(root)
 	}