VSTOP cannot be used when screen is started in detached mode

Hello.

I do have a problem that i would really love to solve.  I can
reproduce it with current ArchLinux (Screen version 4.06.01 (GNU)
10-Jul-17) and FreeBSD 11.1 (iirc 4.05 Dec 16; i cannot compile
screen on FreeBSD easily, utmp still fails there and i have no
time to get over that).

First though i want to thank Thomas Dickey and "jdw" which got me
going[1] on first glance on FreeBSD.  I.e., i have used -A -D -R
since "ever" but now wanted to turn to -a -D -m for
a constellation where i cannot forward ssh-agent from some point,
yet wanted it as the parent of a screen session on target.
This turned out to be an Odysee, but finally searched the internet
and luckily found the mentioned [1].  Because -T and "term xz" and
"TERM=XY screen" did not mean a thing.  Anyway, the solution to
get me going on/to FreeBSD now is, thanks to jdw,

  if command -v screen >/dev/null 2>&1; then
    if screen -list >/dev/null 2>&1; then :; else
      (
      eval "$(tset - -s screen | tail -n+2)"
      exec env SYSSCREENRC=/dev/null TERM=xterm-256color \
        SCREENCAP="${TERMCAP}" LC_ALL=en_US.UTF-8 \
        ssh-agent screen -a -D -m
      ) </dev/null >/dev/null 2>&1 & 
    fi
  fi
  exec /usr/bin/login -f steffen

tset(1) outputs $TERMCAP on FreeBSD.  (And this is different to
Linux target(s), where login is .. not usable for my case)

  ..
        LC_ALL=en_US.UTF-8 ssh-agent screen -a -D -m
  ..
  exec mksh -l

Ok, so i can now attach via "screen -r" and have my three color
vim(1), and it seems good enough to work in a startup script.

  [1] https://unix.stackexchange.com/questions/67450/gnu-screen-strange-termcap-when-using-d-m/273936

However, we, that is my mailer, for which i have written the
termcap/terminfo etc. interface, and me, still do not get all the
key-bindings that we would get with -A -D -R (-kLFT kRIT kUP kDN),
but what i really do not get is that ^S (a.k.a. $'\cS') does not
work in this context, neither on FreeBSD nor on Linux!
The Mailx Line Editor does (tty.c, line ~942):

   /* Enable ^\, ^Q and ^S to be used for key bindings */
   tiosp->c_cc[VQUIT] = tiosp->c_cc[VSTART] = tiosp->c_cc[VSTOP] = '\0';
   tiosp->c_iflag &= ~(ISTRIP | IGNCR);
   tiosp->c_lflag &= ~(ECHO /*| ECHOE | ECHONL */| ICANON | IEXTEN);

so VSTOP is explicitly disabled.  stty(1) shows "stop = ^S;".

Can i improve that some more?  Why *does* screen not look at $TERM
when starting in detached mode, nor react upon -T / "term" when
set explicitly?  Anything is just as it should in -A -D -R mode!

Thanks in advance, and have a nice (rest) weekend!

Ciao!

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)