hwaddr templating

This change introduce mac address templating.

By setting lxc.network.hwaddr to something like fe:xx:xx:xx:xx:xx
each "x" will be replaced by a random value.
If less significant bit of first byte is "templated", it will be
set to 0.

This chage introduce also a common randinit() function that could be
used to initialize random generator.

Signed-off-by: gza <lxc@zitta.fr>