hostapd: Add noscan mode.

Applies OpenWRT's noscan patch to hostapd and the relevant option to
the hostapd module.

This noscan patch adds a new `noscan` option allowing us to create
some overlapping BSSs in HT40+/- mode.

Note: this option is disabled by default, we leave this up to the end
user whether it should be enabled or not.

Not being able to create those overlapping BSSs is basically
preventing us to use 802.11n in any urban area where chances to
overlap with another SSID are extremely high.

The patch we are using is a courtesy of the openwrt team and is
applied to the defaul hostapd package in both OpenWRT and Archlinux.