[RFC] Cleanup os_unix.h

Removed not defined prototypes in `os_unix.h`:  
  * reset_signals, vim_handle_signal:
    signal handling was rewritten, not defined anywhere

  * related to x clipboard handling, not defined anywhere:
    * {setup,start,stop,clear}_xterm_clip
    * stop_xterm_trace
    * clip_xterm_{own_selection,lose_selection,request_selection,set_selection}

  * related to XSMP (x session management protocol):
    * xsmp_{handle_requests,init,close}