dhcp: log messages about invalid DHCP options

Log messages when invalid DHCP options are found. For example:

  <info> dhcp4 (eth0): error parsing DHCP option 6 (domain_name_servers): address 0.0.0.0 is ignored
  <info> dhcp4 (eth0): error parsing DHCP option 12 (host_name): '.example.com' is not a valid DNS domain
  <info> dhcp4 (eth0): error parsing DHCP option 26 (interface_mtu): value 60 is smaller than 68

!1225
56 jobs for bg/dhcp-log-invalid-options in 35 minutes and 2 seconds (queued for 5 seconds)