hostapd: Detect bridge interface automatically

hostapd: Detect bridge interface automatically

This makes the bridge parameter unnecessary for cases where the interface
is already in a bridge and sysfs is mounted to /sys so that the detection
code works.

For nl80211, the bridge parameter can be used to request the AP
interface to be added to the bridge automatically (brctl may refuse to
do this before hostapd has been started to change the interface mode).
If needed, the bridge interface is also created.