Correct implicit username for MSYS2 eg. xs @server.com

Signed-off-by: Russ Magee <rmagee@gmail.com>

xs/xs.go

@@ -769,7 +769,7 @@ func main() {
 	var uname string
 	if remoteUser == "" {
 		u, _ := user.Current() // nolint: gosec
-		uname = u.Username
+		uname = localUserName(u)
 	} else {
 		uname = remoteUser
 	}
@@ -1037,6 +1037,19 @@ func main() {
 	exitWithStatus(int(rec.Status()))
 }
 
+// currentUser returns the current username minus any OS-specific prefixes
+// such as MS Windows workgroup prefixes (eg. workgroup\user).
+func localUserName(u *user.User) string {
+	if u == nil {
+		log.Fatal("null User?!")
+	}
+
+	// WinAPI: username may have CIFS prefix %USERDOMAIN%\
+	userspec := strings.Split(u.Username, `\`)
+	username := userspec[len(userspec)-1]
+	return username
+}
+
 func restoreTermState(oldState *xs.State) {
 	_ = xs.Restore(os.Stdin.Fd(), oldState) // nolint: errcheck,gosec
 }