[AD] abort on exit in X port

[Peter Wang <tjaden@xxxxxxxxxx>]

I've been meaning to bring this up but kept forgetting.  Is anyone else 
getting SIGABRTs when Allegro programs exit, in the X port?  It doesn't 
always happen, but it's not uncommon either.  It looks like it was 
introduced in the latest WIP, although I'm not sure (will investigate 
further as required).

A sample backtrace: (with exalpha, but happens for others too)

Program received signal SIGABRT, Aborted.
_xwin_close_display () at src/x/xwin.c:345
345           XCloseDisplay(dpy);
(gdb) bt
#0  _xwin_close_display () at src/x/xwin.c:345
#1  0x08081859 in _xwin_sysdrv_exit () at src/x/xsystem.c:226
#2  0x0804c045 in allegro_exit () at src/allegro.c:408
#3  0x0804bd54 in allegro_exit_stub () at src/allegro.c:284
#4  0x401b665f in exit () from /lib/libc.so.6
#5  0x401a093e in __libc_start_main () from /lib/libc.so.6
#6  0x0804b2c1 in _start ()

Peter