astraceroute: Don't call device_mtu() twice in a row

Store the result in len_rcv right away and reuse it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
xio: Mention name of file in open_or_die{,_m} error message

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
astraceroute: Fix 'unkown' typo in a few other places

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>