Added support for setting channel mask for scan requests
('iwpriv wlan0 scan_channels 0x00ff' masks scans to use channels 1-8).
Signed-off-by: NJouni Malinen <jkmaline@cc.hut.fi>
Signed-off-by: NJeff Garzik <jgarzik@pobox.com>

Includes minor cleanups from Adrian Bunk <bunk@stusta.de>.