dhcpra: refactor return value of dhcpra_handle_boot{request,reply}

This change makes it more obvious what the return value means, and exposes an
inconsistency with the comment for dhcpra_handle_pkt. The comment says that an
unsupported VLAN codition results in a packet-in to the controller, but the
code clearly does not.

No functional change.
dhcpra: send "unsupported VLAN" packets to the controller and downgrade error message

DHCP packets can come to the CPU for other reasons besides needing processing
from the DHCP agent. The DHCP agent should not drop them.
dhcpra: skip DHCP processing for invalid and reserved VLANs
dhcpra: add a define for SYSTEM_VLAN
Merge into master from pull request #60:
Don't drop DHCP packets on unsupported VLANs (BVS-2176) (https://github.com/floodlight/switchlight-common/pull/60)