fastd: generate the key from urandom

We do not use encrypted tunnels, so we can use urandom generating the keys to prevent blocking due to low entropy.

Signed-off-by: Robert Langhammer <rlanghammer@web.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>