luci-base: let luci.sys.net.devices() return all netdevs

The previous implementation of the function only returned ethernet
interfaces because it relied on the AF_PACKET family entries returned
by getifaddrs().

Change the function to simply collect all interface names it sees in
order to avoid missing tunnel interfaces.

Fixes FS#917.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>