UNIXSocket#addr and UNIXSocket#path do not work on NetBSD (< 8)

[tsahara]

% mruby -e 'c = UNIXSocket.open("/tmp/foo"); p  c.addr'
trace:
        [0] -e:1
-e:1:invalid sockaddr (too short) (SocketError)

% mruby -e 'c = UNIXSocket.open("/tmp/foo"); p  c.path'
trace:
        [0] -e:1
-e:1:invalid sockaddr (too short) (SocketError)

That's because getsockname(unixsock) returns two-bytes sockaddr (is it a sockaddr?). See BUGS section of getsockname(2).