Give a name to the TUI SingleKey keymap

Give a name to the TUI SingleKey keymap

Readline 8.0 has a feature that lets an application name a keymap.
This in turn makes it simpler for users to bind keys in keymaps in
their .inputrc.

This patch gives a name to the TUI SingleKey keymap, so that
additional bindings can be made there.  For example:

    $if gdb
    set keymap SingleKey
    "X": "echo hello\\n\n"
    $endif

The call to rl_initialize, in tui_initialize_readline, had to be
removed so that .inputrc was not read too early.  Note that Readline
explicitly documents that this call is not needed.

gdb/ChangeLog
2019-09-18  Tom Tromey  <tom@tromey.com>

	* NEWS: Add entry.
	* tui/tui.c (tui_initialize_readline): Set name of keymap.  Do not
	call rl_initialize.
	(tui_enable): Do not call rl_initialize.

gdb/doc/ChangeLog
2019-09-18  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Editing): Document readline application name.
	(TUI Single Key Mode): Document TUI SingleKey keymap name.