Bug 928362: Support ad-hoc mode for Wifi, r=vchang, r=mrbkap

This patch adds support for Wifi networks that run in ad-hoc
mode. The network attributes 'frequency' and 'mode' are required
by wpa_supplicant to connect to an ad-hoc network.

'Frequency' is just the frequency of the channel in use. 'Mode'
is the network's mode, with 0 being infrastructure mode, 1 being
IBSS mode (aka ad-hoc mode), and 2 being access-point mode.

If ad-hoc mode is not supported on your device, you can set the
environment property 'ro.moz.wifi.ibss_supported' to false to
disable it in Gecko.