Import dhcpcd-6.0.4 with the following changes:
* hostname is not stamped on anymore if already set
* hostname will be set to a FQDN when possible as per RFC4702 section 3.1
* a domain is derived from the FQDN if no domain option is set
* add new hostname_short command to send a short hostname for DDNS
* hostname_fqdn is now documented, along with a new server setting and the
  potential problems associated with it
* the FQDN option is no longer sent by default for DHCPv4 messages