use pthread for mplex detach polling for better portability

src/term_mplex.h

@@ -27,6 +27,9 @@
    yes, it initializes a timer which periodically checks the attached/detached
    state of the terminal and updates away status accordingly.
  */
-void init_mplex_away_timer (Tox *m);
+int init_mplex_away_timer (Tox *m);
+
+void lock_status ();
+void unlock_status ();
 
 #endif /* #define TERM_MPLEX_H */