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...).

test/usage_unix.go

@@ -12,9 +12,10 @@ package integration
 import (
 	"log"
 	"os"
-	"runtime"
 	"syscall"
 	"time"
+
+	"github.com/syncthing/syncthing/lib/build"
 )
 
 func printUsage(name string, proc *os.ProcessState, total int64) {
@@ -22,7 +23,7 @@ func printUsage(name string, proc *os.ProcessState, total int64) {
 		mib := total / 1024 / 1024
 		log.Printf("%s: Utime: %s / MiB", name, time.Duration(rusage.Utime.Nano()/mib))
 		log.Printf("%s: Stime: %s / MiB", name, time.Duration(rusage.Stime.Nano()/mib))
-		if runtime.GOOS == "darwin" {
+		if build.IsDarwin {
 			// Darwin reports in bytes, Linux seems to report in KiB even
 			// though the manpage says otherwise.
 			rusage.Maxrss /= 1024

test/util.go

@@ -25,6 +25,7 @@ import (
 	"time"
 	"unicode"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/rc"
 	"github.com/syncthing/syncthing/lib/sha256"
 )
@@ -174,7 +175,7 @@ func alterFiles(dir string) error {
 
 		// Change capitalization
 		case r == 2 && comps > 3 && rand.Float64() < 0.2:
-			if runtime.GOOS == "darwin" || runtime.GOOS == "windows" {
+			if build.IsDarwin || build.IsWindows {
 				// Syncthing is currently broken for case-only renames on case-
 				// insensitive platforms.
 				// https://github.com/syncthing/syncthing/issues/1787