patch 9.1.0321: Garbled output on serial terminals with XON/XOFF flow control

Problem:  When used terminal with XON/XOFF flow control, vim tries to
          still make CTRL-S mapping available, which results in severe
          screen corruption, especially on large redraws, and even
          spurious inputs (John Tsiombikas)
Solution: Disallow CTRL-S mapping if such terminal is recognized.
          Don't remove IXON from the bitmask inversion.
          (Anton Sharonov)

*** When started like this:

    TERM=vt420 vim

:set termcap

    shows "t_xon=y"

map <C-S> :echo "abc"<CR>

    does nothing (after <C-S> output freezes and subsequent <C-Q>
    unfreezes it)

*** When started like this:

    TERM=xterm vim

:set termcap

    shows "t_xon="

map <C-S> :echo "abc"<CR>

    works (after <C-S> one see "abc" string echo-ed)

fixes: #12674
closes: #14542

Signed-off-by: Anton Sharonov <anton.sharonov@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/optiondefs.h

@@ -3014,6 +3014,7 @@ static struct vimoption options[] =
     p_term("t_8f", T_8F)
     p_term("t_8b", T_8B)
     p_term("t_8u", T_8U)
+    p_term("t_xo", T_XON)
 
 // terminal key codes are not in here