move setuid/setgid into separate functions in platform-dependent files

cmd/yggdrasil/main.go

@@ -376,12 +376,18 @@ func run(args yggArgs, ctx context.Context, done chan struct{}) {
 	// Lower permissions from root to something else, if the user wants to
 	if syscall.Getuid() == 0 {
 		if args.rungid > 0 {
-			fmt.Println("Dropping gid to ", args.rungid)
-			syscall.Setgid(args.rungid)
+			logger.Infoln("Setting gid to:", args.rungid)
+			if err := setgid(args.rungid); err != nil {
+				logger.Errorln("Failed to set gid:", err)
+				return
+			}
 		}
 		if args.runuid > 0 {
-			fmt.Println("Dropping uid to ", args.rungid)
-			syscall.Setuid(args.runuid)
+			logger.Infoln("Setting uid to:", args.runuid)
+			if err := setuid(args.runuid); err != nil {
+				logger.Errorln("Failed to set uid:", err)
+				return
+			}
 		}
 	}
 	logger.Infof("Your public key is %s", hex.EncodeToString(public[:]))

cmd/yggdrasil/setids_other.go

@@ -0,0 +1,13 @@
+// +build !linux,!darwin,!freebsd,!openbsd
+
+package main
+
+import "errors"
+
+func setuid(uid int) error {
+	return errors.New("setting uid not supported on this platform")
+}
+
+func setgid(gid int) error {
+	return errors.New("setting gid not supported on this platform")
+}

cmd/yggdrasil/setids_posix.go

@@ -0,0 +1,13 @@
+// +build linux darwin freebsd openbsd
+
+package main
+
+import "syscall"
+
+func setuid(uid int) error {
+	return syscall.Setuid(uid)
+}
+
+func setgid(gid int) error {
+	return syscall.Setgid(gid)
+}