networkd: rename a few Network object properties to be more like the configuration...

networkd: rename a few Network object properties to be more like the configuration settings

All booleans called dhcp_xyz are now called ".dhcp_use_xyz", to match their respective configuration file settings. This
should clarify things a bit, in particular as there is a DHCP hostname that was previously called just ".hostname"
because ".dhcp_hostname" was already existing as a bool. Since this confusion is removed now because the bool is called
".dhcp_use_hostname", the string field is now renamed to ".dhcp_hostname".