networkd: improve logging
networkd: link - only save original MTU when necessary
networkd: dhcp - only set the MTU option once

Only set MTU request when creating the dhcp client, not every time it is restarted.
sd-dhcp-client/networkd: add transient hostname support
sd-dhcp-client: refactor DNS support

Rather than keeping an array of pointers to addresses, just keep an array of addresses.
sd-dhcp-client: minor style fixes

Also introduce a cleanup macro for DHCPLease
networkd: resolv.conf - extend the comment a bit