bin/dev: set RCSHELL to /dev/null on default case

When no shell is detected, set `RCSHELL` variable to point to
/dev/null.
This avoid weird error messages.

bin/dev.sh

@@ -29,6 +29,7 @@ case $SHELL in
 	;;
 *)
 	echo "SHELL don't supported.  Try using BASH or ZSH, or manually."
+	RCSHELL="/dev/null"
 	;;
 esac