refact, linux headless option, debug

Signed-off-by: fufesou <shuanglongchen@yeah.net>

src/platform/linux_desktop_manager.rs

@@ -109,7 +109,7 @@ pub fn try_start_desktop(_username: &str, _passsword: &str) -> String {
             // No need to verify password here.
             return "".to_owned();
         }
-        if username.is_empty() {
+        if !username.is_empty() {
             // Another user is logged in. No need to start a new xsession.
             return "".to_owned();
         }