Fix incorrect netmasks being passed via rtsock.

Since radix has been ignoring sa_family in passed sockaddrs,
no one ever has bothered filling valid sa_family in netmasks.
Additionally, radix adjusts sa_len field in every netmask not to
compare zero bytes at all.

This leads us to rt_mask with sa_family of AF_UNSPEC (-1) and
arbitrary sa_len field (0 for default route, for example).

However, rtsock have been passing that rt_mask intact for ages,
requiring all rtsock consumers to make ther own local hacks.
We even have unfixed on in base:

do `route -n monitor` in one window and issue `route -n get addr`
for some directly-connected address. You will probably see the following:

got message of size 304 on Thu May  8 15:06:06 2014
RTM_GET: Report Metrics: len 304, pid: 30493, seq 1, errno 0, flags:<UP,DONE,PINNED>
locks:  inits:
sockaddrs: <DST,GATEWAY,NETMASK,IFP,IFA>
 10.0.0.0 link#1 (255) ffff ffff ff em0:8.0.27.c5.29.d4 10.0.0.92
_________________^^^^^^^^^^^^^^^^^^

after the change:

got message of size 312 on Thu May  8 15:44:07 2014
RTM_GET: Report Metrics: len 312, pid: 2895, seq 1, errno 0, flags:<UP,DONE,PINNED>
locks:  inits:
sockaddrs: <DST,GATEWAY,NETMASK,IFP,IFA>
 10.0.0.0 link#1 255.255.255.0 em0:8.0.27.c5.29.d4 10.0.0.92
_________________^^^^^^^^^^^^^^^^^^

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