Assume natural alignment for IP & ether addrs

Assume natural alignment for IP addresses and ethernet addresses in
the critical path.  By doing so, we can compare and copy via direct
4-byte load / store rather than either calling memcmp()/memmove(), or
doing hand-unrolled byte-by-byte operations.

This gains about 750Mb/s on a netperf TCP receive workload with a
10GbE NIC in a low-end x86_64.

Signed-off-by: Andrew Gallatin <gallatin@google.com>