Improve the seed of the pseudo-random number generator

When (re)starting a couple of virtual machines (about 10) on the same host
it frequently happens that some of them use the same DHCP transaction ID.

This problem can be solved by combining the current timer ticks and the
least significant bits of the hardware address(es) to get a better
randomness for the seed.

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>