This patch reflects eventual allocation errors in a more specific way to
the caller of 'alloc_aligned', in particular out-of-metadata and
out-of-memory are considered as different conditions.
Related to issue #526.
By adding a "mac=XX:XX:XX:XX:XX:XX" attribute/value pair to the nic_bridge's
configuration one can define the first MAC address from which the nic_brigde
will allocate MACs for it's clients. Note: that the least relevant byte will
be ignored, and ranges from 0-255. Fixes#424.
