Use the terminal's alternate screen for a full-window pane when clear-on-attach is off - #5580

…-on-attach is off

With clear-on-attach off the client terminal keeps its own scrollback on
the primary screen. A full-screen application in a pane, such as less or
vim, then scrolls that scrollback away: tmux paints the pane's alternate
screen with line feeds on the terminal's primary screen.

When the active pane fills the window, is not in a mode, and is in its
alternate screen, emit smcup to the client terminal before redrawing and
rmcup when the pane leaves it. The application draws on the terminal's
alternate screen and the terminal restores its scrollback on exit.

- tmux.h: TTY_ALTSCREEN flag recording that a pane put the terminal in
  its alternate screen.
- server-client.c: reconcile the terminal's alternate screen with the
  pane's in server_client_check_redraw() after the tty is unfrozen and
  before the redraw. Start the synchronized update before the switch so
  it and the redraw are one frame; leave the ACS character set first
  since the terminal saves it with the cursor; redraw everything with
  server_redraw_client() because the new screen has no status line or
  overlay. Require both smcup and rmcup so a terminal with one but not
  the other is never put on a screen it cannot leave.
- tty.c: leave the alternate screen in tty_stop_tty() if TTY_ALTSCREEN is
  set so a detach or suspend does not leave the terminal in it; clear the
  flag in tty_start_tty().
- tmux.1, options-table.c: describe the behaviour under clear-on-attach.

No change when clear-on-attach is on, when the terminal lacks smcup or
rmcup, in a split, or for applications that do not use the alternate
screen.