- Regularize expression evaluation calls so that they are all passed a pointer

- Regularize expression evaluation calls so that they are all passed a pointer
  to the incoming packet, if any, the current lease, if any, the parsed
  incoming options, and the in-scope configuration file options.
- Incoming packets are now reference counted, so that the callee can save
  them for later expression evaluation (e.g., in dhcp_reply).