Fix various default.exp failures

[law@redhat.com]

default.exp has been intermittently failing on my HPs.  Not surprisingly the
problem is we're using uninitialized memory.

In my case, the problems show up (sometimes) when a "info set" command
is issued.  Typically, the problem shows up when we get to the 
"remotelogfile" entry in the set/show command list.

Basically we never initialize the "pre_show_hook" field in the cmd_list_element
structure for new commands we create via add_set_cmd.

Usually the value is zero and all is well, but sometimes the pre_show_hook
is nonzero (ie garbage).  Consider this code in do_setshow_command and what
happens if pre_show_hook is a nonzero garbage value:

      /* Possibly call the pre hook.  */
      if (c->pre_show_hook)
        (c->pre_show_hook) (c);

Yup, we make an indirect call to some unpredictable location which usually
results in a segfault.

The fix is trivial.  Initialize the pre_show_hook when we add the new command.

	* cli/cli-decode.c (add_cmd): Initialize pre_show_hook in
	the new command.

Index: cli/cli-decode.c
===================================================================
RCS file: /cvs/cvsfiles/devo/gdb/cli/cli-decode.c,v
retrieving revision 1.5
diff -c -3 -p -r1.5 cli-decode.c
*** cli-decode.c	2001/10/01 01:42:05	1.5
--- cli-decode.c	2001/12/20 17:06:22
*************** add_cmd (char *name, enum command_class 
*** 90,95 ****
--- 90,96 ----
    c->doc = doc;
    c->flags = 0;
    c->replacement = NULL;
+   c->pre_show_hook = NULL;
    c->hook_pre  = NULL;
    c->hook_post = NULL;
    c->hook_in = 0;