netmgr: add isc_nm_routeconnect()

netmgr: add isc_nm_routeconnect()

isc_nm_routeconnect() opens a route/netlink socket, then calls a
connect callback, much like isc_nm_udpconnect(), with a handle that
can then be monitored for network changes.

Internally the socket is treated as a UDP socket, since route/netlink
sockets follow the datagram contract.