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>
diff --git a/src/term.c b/src/term.c
index 8aa86ce..7e39b03 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1798,6 +1798,8 @@
 	T_DA = (char_u *)"y";
     if ((T_UT == NULL || T_UT == empty_option) && tgetflag("ut") > 0)
 	T_UT = (char_u *)"y";
+    if ((T_XON == NULL || T_XON == empty_option) && tgetflag("xo") > 0)
+	T_XON = (char_u *)"y";
 
     /*
      * get key codes