iwinfo: avoid creating tmp.* ifaces for scanning

If the iface to scan on already is in ad-hoc, station or monitor mode
then do not spawn a temporary iface.

Also preventively disable IPv6 on temporary ifaces before bringing them
up to avoid potential security issues.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41830 3c298f89-4303-0410-b956-a3cf2f4a3e73