Support matching and modifying IP TTL.

Support matching and modifying IP TTL.

Add support matching the IPv4 TTL and IPv6 hop limit fields.  This
commit also adds support for modifying the IPv4 TTL.  Modifying the IPv6
hop limit isn't currently supported, since we don't support modifying
IPv6 headers.

We will likely want to change the user-space interface, since basic
matching and setting the TTL are not generally useful.  We will probably
want the ability to match on extraordinary events (such as TTL of 0 or 1)
and a decrement action.

Feature #8024

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>