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...).

cmd/syncthing/main.go

@@ -199,7 +199,7 @@ func defaultVars() kong.Vars {
 	// Windows, the "default" options.logFile will later be replaced with the
 	// default path, unless the user has manually specified "-" or
 	// something else.
-	if runtime.GOOS == "windows" {
+	if build.IsWindows {
 		vars["logFile"] = "default"
 	} else {
 		vars["logFile"] = "-"