plan9 picked an ipv4 local address for outgoing ipv6 connection.
machine picked local=::ffff:10.192.254.99 for remote=2001:db8::1.
incoming connections from that machine worked fine.

term% cat /net/iproute
0.0.0.0         /96  10.192.254.33   4    none   -
10.0.0.0        /128 10.0.0.0        4b   ifc    -
10.192.254.0    /120 10.192.254.0    4i   ifc    0
10.192.254.0    /128 10.192.254.0    4b   ifc    -
10.192.254.99   /128 10.192.254.99   4u   ifc    0
10.192.254.255  /128 10.192.254.255  4b   ifc    -
10.255.255.255  /128 10.255.255.255  4b   ifc    -
255.255.255.255 /128 255.255.255.255 4b   ifc    -
2001:db8::      /64  2001:db8::      6i   ifc    0
fe80::          /64  fe80::          6i   ifc    -
ff02::          /16  ff02::1         6m   ifc    0
ff02::1         /128 ff02::1         6m   ifc    0
2001:db8::2d0:59ff:fecf:73a5 /128 2001:db8::2d0:59ff:fecf:73a5 6u   ifc    0
fe80::2d0:59ff:fecf:73a5 /128 fe80::2d0:59ff:fecf:73a5 6u   ifc    0
ff02::1:ffcf:73a5 /128 ff02::1:ffcf:73a5 6m   ifc    0

this patch threats ipv4 addresses as unknown so they dont get
used. not sure if this function is the right place todo it or if we
should test the ifc flags for v4 instead.
probably not the right place to fix it?