[RFA] Ensure class_tui is listed in the output of "help" giving the list of classes.

[Philippe Waroquiers]

Before this change, "help" was not showing the TUI class.
With this change:
  (gdb) help
  ...
  support -- Support facilities.
  text-user-interface -- TUI is the GDB text based interface.
  tracepoints -- Tracing of program execution without stopping the program.
  ...
  (gdb) help text-user-interface
  TUI is the GDB text based interface.
  In TUI mode, GDB can display several text windows showing
  the source file, the processor registers, the program disassembly, ...

  List of commands:

  + -- Scroll window forward.
  ...

Note that we cannot use "tui" for the fake class command name, as "tui"
is a command.

gdb/ChangeLog

YYYY-MM-DD  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* command.h: Add comment giving the name of class_tui.
	* cli/cli-cmds.c (_initialize_cli_cmds): If TUI defined,
	create the fake command for the help for class_tui.
---
 gdb/cli/cli-cmds.c | 6 ++++++
 gdb/command.h      | 2 +-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index eb6e32b046..fdc8758bcd 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -2150,6 +2150,12 @@ Variable lookups are done with respect to the selected frame.\n\
 When the program being debugged stops, gdb selects the innermost frame.\n\
 The commands below can be used to select other frames by number or address."),
 	   &cmdlist);
+#ifdef TUI
+  add_cmd ("text-user-interface", class_tui,
+	   _("TUI is the GDB text based interface.\n\
+In TUI mode, GDB can display several text windows showing\n\
+the source file, the processor registers, the program disassembly, ..."), &cmdlist);
+#endif
   add_cmd ("running", class_run, _("Running the program."), &cmdlist);
 
   /* Define general commands.  */
diff --git a/gdb/command.h b/gdb/command.h
index 04a380cba4..32b5b35b0c 100644
--- a/gdb/command.h
+++ b/gdb/command.h
@@ -64,7 +64,7 @@ enum command_class
   class_bookmark,
   class_obscure,     /* obscure */
   class_maintenance, /* internals */
-  class_tui,
+  class_tui,         /* text-user-interface */
   class_user,        /* user-defined */
 
   /* Used for "show" commands that have no corresponding "set" command.  */
-- 
2.20.1