SystemMonitor: Use kill(pid,0) when checking for kill permission

We can use kill(pid,0) to check for kill permissions instead of relying
on file path access. Using 0 as signal does error checking but does not
send a signal.

Userland/Applications/SystemMonitor/main.cpp

@@ -87,8 +87,8 @@ private:
 
 static bool can_access_pid(pid_t pid)
 {
-    auto path = String::formatted("/proc/{}", pid);
+    int rc = kill(pid, 0);
-    return access(path.characters(), X_OK) == 0;
+    return rc == 0;
 }
 
 int main(int argc, char** argv)