pan: baseUDPConn still sends packets to the fix port for endhost within the local AS

[JordiSubira]

Currently, baseUDPConn is sending packets to the fix port for endhost within the local AS

scion-apps/pkg/pan/raw.go

Line 71 in 55667b4

nextHop = netip.AddrPortFrom(dst.IP, underlay.EndhostPort)

We should replicate the behavior of snet.NewCookedConn() somehow since we are opening directly a

scion-apps/pkg/pan/udp_listen.go

Line 88 in 55667b4

conn, err := sn.OpenRaw(ctx, net.UDPAddrFromAddrPort(local))

. Maybe using "NewCookedConn" is good enough...