gateway: determine default local IP

[matzf]

Determine default local IP for SIG as documented in config sample,
analogous to old SIG implementation.
Instead of copying the findDefaultLocalIP logic again, move to
addrutil.DefaultLocalIP library function.

Additionally includes losely related small cleanup in sciond.TopologyQuerier. (was included in 3938).

Closes #3933.

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This change is 

[oncilla]

Reviewed 6 of 6 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @matzf and @scrye)

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go/lib/snet/addrutil/addrutil.go, line 103 at r1 (raw file):

// This returns a sensible but arbitrary local IP. In the general case the
// local IP would depend on the next hop of selected path. This approach will
// not work in more complicated setups where e.g. different network interface

s/interface/interfaces

[matzf]

Reviewable status: 5 of 6 files reviewed, 1 unresolved discussion (waiting on @oncilla and @scrye)

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go/lib/snet/addrutil/addrutil.go, line 103 at r1 (raw file):

Previously, Oncilla (Dominik Roos) wrote…

s/interface/interfaces

Well spotted 👀 Done.

[oncilla]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @scrye)

[scrye]

Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @matzf)

a discussion (no related file):
I was thinking about adding a request for documenting the behavior in the Gateway manual, but I think I'll take care of that afterwards (the behaviors for some configuration options are quite complex, and need a lot of documentation).

Found a few nits here and there, but otherwise we can merge this.

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go/lib/snet/addrutil/addrutil.go, line 99 at r2 (raw file):

}

// DefaultLocalIP returns _a_ IP of this host in the local AS.

Nit: "a" -> "an"

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go/lib/snet/addrutil/addrutil.go, line 107 at r2 (raw file):

//
// This is a simple workaround for not being able to use wildcard addresses
// with snet. Once a available, a wildcard address should be used instead and

Remove "a" before "available".

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go/lib/snet/addrutil/addrutil.go, line 108 at r2 (raw file):

// This is a simple workaround for not being able to use wildcard addresses
// with snet. Once a available, a wildcard address should be used instead and
// this should simply be removed.

Remove "simply".

[matzf]

Reviewable status: 5 of 6 files reviewed, 3 unresolved discussions (waiting on @oncilla and @scrye)

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go/lib/snet/addrutil/addrutil.go, line 107 at r2 (raw file):

Previously, scrye (Sergiu Costea) wrote…

Remove "a" before "available".

Done.

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go/lib/snet/addrutil/addrutil.go, line 108 at r2 (raw file):

Previously, scrye (Sergiu Costea) wrote…

Remove "simply".

Done.

[oncilla]

Reviewed 1 of 1 files at r3.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @scrye)

[scrye]

Reviewable status: complete! all files reviewed, all discussions resolved