Handle recent ifconfig(8) outputs in the miniroot installation script.

- Remove netmask slash notation for IP addresses,
  which has been changed between NetBSD 7.x and 8.0:
  http://cvsweb.netbsd.org/bsdweb.cgi/src/sbin/ifconfig/af_inet.c#rev1.24
- Ignore inet6 entries, which miniroot scripts don't support

Should be pulled up to netbsd-9 and netbsd-8.

(tsutsui)