Follow dhclient -> dhcpleased switch, reduce supported DHCP statements

Follow dhclient -> dhcpleased switch, reduce supported DHCP statements

dhcpleased(8) does not support every statement dhclient(8) does, so reflect
the status quo simplify things to the smaller set of statements that still
suffice to use autoinstall(8).

While here, speak of dhcp-options(5) and "DHCP lease" instead of
dhcpd.conf statements and the installer's specific DHCP client/parser.

OK florian