The console server should broadcast a bell event on one
virtual console to all virtual consoles.  This can then be
used by the console clients to notify the user of the bell
event in the other consoles (for example by ringing the bell,
or by a screen-like message "bell in window NR".

For this, a flag in cons_change_t what could be used, and if
that flag is on, the second word that is still free could
be used to contain the VC number (hurd/console.h):

typedef union
{
  struct
  {
    uint32_t start;
    uint32_t end;
  } matrix;
  struct
  {
    uint32_t unused;
    uint32_t vc;
  } extern_bell_audible;
  struct
  {
    uint32_t cursor_pos : 1;
    uint32_t cursor_status : 1;
    uint32_t screen_cur_line : 1;
    uint32_t screen_scr_lines : 1;
    uint32_t bell_audible : 1;
    uint32_t bell_visible : 1;
    uint32_t flags : 1;
    uint32_t extern_bell_audible : 1;
    uint32_t _unused : 23;
    uint32_t not_matrix : 1;
    /* Here are 32 more unused bits.  */
  } what;
} cons_change_t;