add check to detect OS (Debian) where su-bruteforce cannot be used

[leoCottret]

Hello, I noticed during a CTF that su-bruteforce didn't work for some reason.
After digging a bit, it's because of the OS.
su from Debian will not authorize the use of su without a terminal

I found the relevant code here:
https://sources.debian.org/src/shadow/1:4.5-1.1/src/su.c/?hl=720#L720
I think this problem is still relevant
https://sources.debian.org/src/shadow/1%3A4.8.1-1/src/su.c/#L721

Since the current code discard the stderr, the current behavior is a false negative.

  trysu=`echo "$PASSWORDTRY" | timeout $TIMEOUTPROC su $USER -c whoami 2>/dev/null` 
  if [ "$trysu" ]; then
    echo "  You can login as $USER using password: $PASSWORDTRY" | sed "s,.*,${C}[1;31;103m&${C}[0m,"
    exit 0;
  fi

So I added a check, a message and stop the script in case it can't be used against this type of OS

PS: the relevant CTF
https://tryhackme.com/room/jackofalltrades
Debian 3.16.7

EDIT:
I noticed that linpeas already contains a check to see if su exists: peass-ng/PEASS-ng#363
and a check on the "must be run from a terminal" that already existed

So I ported them instead.