DHCP client support

[rjarry]

Description

New GR_IFACE_F_DHCP flag for interfaces that triggers a periodic send of DHCP discover packets until a response is received. Upon reception, the response should be parsed and the related configuration should be applied (addresses, routes, search domain, hostname) with appropriate settings (e.g. GR_NH_ORIGIN_DHCP).

Since grout does not terminate TCP/UDP connections, the search domain and hostname DHCP options should only be stored as metadata and trigger events when they change (e.g.: GR_EVENT_HOSTNAME_CHANGE and GR_EVENT_SEARCH_DOMAIN_CHANGE).

Context and Use Cases

Most cases will see grout with static addresses and configuration, but it could be useful to support automatic configuration.