- Do not respond with NAK if ciaddr is set and imputed network doesn't

- Do not respond with NAK if ciaddr is set and imputed network doesn't
  match, since ciaddr means client is unicasting using IP routing.
- Support DHCPINFORM even on unknown networks.
- Fix up some invocations of evaluate_option_cache that used
  post-scope-execution option state rather than packet option state as
  input for evaluations of options from post-scope-execution option
  state.
- Add support for site-defined option spaces.
- Compute boot file server prior to freeing options used in computing
  it.   May fix a core dump that has been reported but that I haven't
  seen.
- Make pool scope less specific than class scope.
- Fix some invocations of execute_statements_in_scope that were
  incorrectly passing output options where they should have passed
  input options.
- Enforce maximum lease length after applying default lease time.
- Compute value of bootfile server IP address in ack_lease instead of
  dhcp_reply, so that expressions using contents of packet can work.