Fix the processing of Meta-key commands in TUI

Fix the processing of Meta-key commands in TUI

This patch fixes the annoying bug where key sequences such as Alt_F or
Alt_B (go forward or backwards by a word) do not behave promptly in TUI.
You have to press a third key in order for the key sequence to register.

This is mostly ncurses' fault.  Calling wgetch() normally causes ncurses
to read only a single key from stdin.  However if the key read is the
start-sequence key (^[ a.k.a. ESC) then wgetch() reads TWO keys from
stdin, storing the 2nd key into an internal FIFO buffer and returning
the start-sequence key.  The extraneous read of the 2nd key makes us
miss its corresponding stdin event, so the event loop blocks until a
third key is pressed.  This explains why such key sequences do not
behave promptly in TUI.

To fix this issue, we must somehow compensate for the missed stdin event
corresponding to the 2nd byte of a key sequence.  This patch achieves
this by hacking  up the stdin event handler to conditionally execute the
readline callback multiple times in a row.  This is done via a new
global variable, call_stdin_event_handler_again_p, which is set from
tui_getc() when we receive a start-sequence key and notice extra pending
input in the ncurses buffer.

Tested on x86_64-unknown-linux-gnu.

gdb/ChangeLog:

	* event-top.h (call_stdin_event_handler_again_p): Declare.
	* event-top.c (call_stdin_event_handler_again_p): Define.
	(stdin_event_handler): Use it.
	* tui/tui-io.c (tui_getc): Prepare to call the stdin event
	handler again if there is pending input following a
	start sequence.