gdb: New 'tui enable' and 'tui disable' commands.

gdb: New 'tui enable' and 'tui disable' commands.

Add new commands to specifically enable and disable tui mode.  This is
in addition to the readline bindings, but might be easier for a user to
discover if they accidentally end up in tui mode.

gdb/ChangeLog:

	* NEWS: Mention 'tui enable' and 'tui disable'.
	* tui/tui.c (tui_enable_command): New function.
	(tui_disable_command): New function.
	(_initialize_tui): New function.

gdb/doc/ChangeLog:

	* gdb.texinfo (TUI): Include 'tui enable' in the introduction.
	(TUI Commands): Add 'tui enable' and 'tui disable' details.