The MTU for the socket interface, if available.
Not available for Java 5.
Note that we don't return the value for the default interface if
we can't find the address. Finding the default interface is hard,
altough we could perhaps just look for the first non-loopback address.
But the MTU of the default route probably isn't relevant.
ia - null ok
0 if Java 5, or if not bound to an address;
limited to range MIN_MTU to LARGE_MTU.
public static int rectify(boolean isIPv6,
min of PeerState.MIN_MTU, max of PeerState.LARGE_MTU,
rectified so rv % 16 == 12 (IPv4)
or rv % 16 == 0 (IPv6)