Fix PR remote/20398: File-IO write always outputs "Quit"
    
    Commit bb7c96deb1a1 ("gdb/remote-fileio.c: Eliminate custom SIGINT
    signal handler") regressed the File-IO support.
    
    Failed output:
    
      (gdb) target remote :8888
      Remote debugging using :8888
      0x00008098 in _start ()
      (gdb) c
      Continuing.
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      Quit
      [Inferior 1 (Remote target) exited normally]
    
    Expected output:
    
      (gdb) target remote :8888
      Remote debugging using :8888
      0x00008098 in _start ()
      (gdb) c
      Continuing.
      i: 0
      i: 1
      i: 2
      i: 3
      i: 4
      i: 5
      i: 6
      i: 7
      i: 8
      i: 9
      [Inferior 1 (Remote target) exited normally]
    
    The problem that the new File-IO quit handler forgets to check the
    quit flag before calling throwing a quit.
    
    gdb/ChangeLog:
    2016-08-05  Pedro Alves  <palves@redhat.com>
    
    	PR remote/20398
    	* remote-fileio.c (remote_fileio_quit_handler): Check the quit
    	flag before calling quit.