Allow setdevname, setspeed and setipaddr to return a fatal error

Add check_prot argument to options_from_file; check permissions
on ~/.ppprc and argument of file option
Process options for tty port (/etc/ppp/options.<ttyname>)
Don't allow loopback or multicast addresses for local or remote
IP address, and don't use these for default local IP address
Don't allow netmask option to reduce the network part of the mask