Allow the --remote_ip config parameter to be a prefix, not just a
single address. The entire remote prefix is routed to the tun device
or remote packetdrill machine.

The goal here is to all tests to use more than one remote IP. The
immediate motivation is testing GRE, where there is a remote IP for
the TCP endpoint and a separate remote IP for the GRE decapsulator.
But eventually we'll also want to allow testing multiple concurrent
TCP connections from multiple remote IPs addresses as well.

Here are examples of the remote prefix you get for various values of
the --remote_ip field. These are chosen for convenience and backwards
compatibility with existing tests that are already using the
--remote_ip parameter.

o Default:

configuration                      remote prefix
------------------------------     -----------------
IPv4, no --remote_ip            -> 192.0.2.0/24   (default)
v4/6, no --remote_ip            -> 192.0.2.0/24   (default)
IPv6, no --remote_ip            -> 2001:DB8::0/32 (default)

o Explicit IP without prefix length (prefix includes all bits):

configuration                      remote prefix
------------------------------     -----------------
IPv4 --remote_ip=10.248.0.3     -> 10.248.0.3/32
v4/6 --remote_ip=10.248.0.3     -> 10.248.0.3/32
IPv6 --remote_ip=2001:db8::3    -> 2001:db8::3/128

o Explicit with prefix length:

configuration                      remote prefix
------------------------------     -----------------
IPv4 --remote_ip=10.248.0.3/16  -> 10.248.0.0/16
v4/6 --remote_ip=10.248.0.3/16  -> 10.248.0.0/16
IPv6 --remote_ip=2001:db8::3/16 -> 2001::0/16

Change-Id: Ibc95c3419d66bcf1bbfd8a1fc7650d0003345ef2

When we sniff an unknown packet, free it and allocate another packet
at the top of the sniffing loop. Previously we were not clearing state
after parsing an unknown packet, so state from the unkbown packet
could linger on, co-mingling with state from the next sniffed packet.


Change-Id: Ia9a330766969b114fd6084a087ff3f0d6b11d9d4

Factor out the packet sniffing loop that's common to local and remote
mode, since it is nearly identical in both cases. This is in
preparation for fixing a bug that is found in both loops, so we can
make the fix in just one place.


Change-Id: Ic9a9b84b66a9f812866b1e29cd7c897f7c1d40cf

Initial commit for the packetdrill network stack testing tool:

  https://code.google.com/p/packetdrill/



Signed-off-by: Neal Cardwell <ncardwell@google.com>