* lib/gdb.exp (gdb_exit): Move the best version of gdb_exit

* lib/gdb.exp (gdb_exit):  Move the best version of gdb_exit
to here.
* config/{mips-gdb.exp, nind-gdb.exp, udi-gdb.exp, vx-gdb.exp}:
Remove various versions of gdb_exit.
* config/vx-gdb.exp:  Remove quit_vxgdb, change a caller to call
gdb_exit.
* config/unix-gdb.exp:  gdb_exit remains here, and should override
the generic version, since it's doing a lot of wierd stuff that
the other versions aren't.  FIXME, fold it in, or abandon this
version.