Let scmp infer local address

[worxli]

This change is 

[matzf]

Related: #3376

[matzf]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @worxli)

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go/tools/scmp/cmn/common.go, line 132 at r1 (raw file):

func ValidateFlags() {
	if Local.Host == nil {
		Local.Host = &addr.AppAddr{L3: addr.HostFromIPStr("127.0.0.1")}

With using localhost IP, we'd just not to receive a reply, unless we happen to run scmp echo on the border router. I think this should use the actual local source address used for sending the SCION packet; using netlink, we can look this up from the kernels routing tables. See also #3376, maybe we can join forces here.

[oncilla]

Fixed by #3691