syscall: fix IPv6 wrong network mask on latest FreeBSD

Looks like latest FreeBSD doesn't set address family identifer
for RTAX_NETMASK stuff; probably RTAX_GENMASK too, not confirmed.
This CL tries to identify address families by using the length of
each socket address if possible.

The issue is confirmed on FreeBSD 9.1.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12332043