packets: Adds ethernet-matching helper functions

With OpenFlow 1.1 requiring arbitrary ethernet match support, it simplifies
other code if we have some extra helper functions. This patch adds
eth_mask_is_exact(mask), eth_addr_bitand(src, mask, dst),
eth_addr_equal_except(a, b, mask) and eth_format_masked(eth, mask, output).

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Ben Pfaff <blp@nicira.com>