gdb: remove cmd_list_element::function::sfunc

I don't understand what the sfunc function type in
cmd_list_element::function is for.  Compared to cmd_simple_func_ftype,
it has an extra cmd_list_element parameter, giving the callback access
to the cmd_list_element for the command being invoked.  This allows
registering the same callback with many commands, and alter the behavior
using the cmd_list_element's context.

From the comment in cmd_list_element, it sounds like at some point it
was the callback function type for set and show functions, hence the
"s".  But nowadays, it's used for many more commands that need to access
the cmd_list_element object (see add_catch_command for example).

I don't really see the point of having sfunc at all, since do_sfunc is
just a trivial shim that changes the order of the arguments.  All
commands using sfunc could just as well set cmd_list_element::func to
their callback directly.

Therefore, remove the sfunc field in cmd_list_element and everything
that goes with it.  Rename cmd_const_sfunc_ftype to cmd_func_ftype and
use it for cmd_list_element::func, as well as for the add_setshow
commands.

Change-Id: I1eb96326c9b511c293c76996cea0ebc51c70fac0

gdb/cli/cli-decode.h

@@ -168,8 +168,8 @@ struct cmd_list_element
      cagney/2002-02-02: This function signature is evolving.  For
      the moment suggest sticking with either set_cmd_cfunc() or
      set_cmd_sfunc().  */
-  void (*func) (struct cmd_list_element *c, const char *args, int from_tty)
-    = nullptr;
+  cmd_func_ftype *func;
+
   /* The command's real callback.  At present func() bounces through
      to one of the below.  */
   union
@@ -179,10 +179,6 @@ struct cmd_list_element
 	 cmd_list_element parameter.  do_simple_func is installed as FUNC, and
 	 acts as a shim between the two.  */
       cmd_simple_func_ftype *simple_func;
-
-      /* If type is set_cmd or show_cmd, first set the variables,
-	 and then call this: */
-      cmd_const_sfunc_ftype *sfunc;
     }
   function;