bsd: Enable auto configuration

bsd: Enable auto configuration

On NetBSD, we should configure some parameters manually out of hostapd
like below.

  ifconfig ath0 mediaopt hostap
  ifconfig ath0 mode 11g
  ifconfig ath0 chan 6

This patch does these automatically. Maybe there will be some
objections, like "hardware configuration is not hostapd/wpa_supplican's
work". So I will write the reasons why I made this patch.

1. For usability.
2. The first command fails when previous state is adhoc. This patch is
free from previous state.
3. Some driver wrappers configure these automatically (like nl80211).
4. I have wasted time trying to find out these command were needed :(