Set interface with longest prefix in DHCP & DHCPv6 lease

- With nested prefixes reside on different interfaces of single host

  (e.g., in 6to4, 2002::/16 on WAN and 2002:<IPv4>:<subnet>::/64 on LAN),

  current matching mechanism might return the interface with shorter prefix

  length instead of the longer one, if it appears later in the netlink message.

Signed-off-by: Lung-Pin Chang <changlp@cs.nctu.edu.tw>
Use event system to re-send query on new route. Tidies module boundaries.
Tidy up previous commit.
Add --dns-loop-detect feature.
Check all servers loopiness, when any subset is changed.
Missed update of DHCP lease datastructure.
GetLoopServers Dbus method.
Mention name in systemd.