Check if fbterm is installed and use $TTY variable

plugins/fbterm/fbterm.plugin.zsh

 # start fbterm automatically in /dev/tty*
 
-if [[ $(tty|grep -o '/dev/tty') = /dev/tty ]] ; then
-	fbterm
-	exit
+if (( ${+commands[fbterm]} )); then
+	if [[ "$TTY" = /dev/tty* ]] ; then
+		fbterm
+		exit
+	fi
 fi