gluon-mesh-babel: implement neighbour check

[AiyionPrime]

This lacks a check for has_default_gw4 or the decision, that babel can't give suitable response.

Resolves #2228.

[neocturne]

Suggested change

	out=$(echo dump | nc ::1 33123 | grep "add neighbour" 2>/dev/null) && [ -n "$out" ]
	echo dump | nc ::1 33123 | grep -qF 'add neighbour'

As we're only interested whether there is matching output or not, grep -q should be used. Adding -F makes it more efficient, as it doesn't interpret the pattern as a regex.

[AiyionPrime]

I still lack a node/network to test this on;
One possible issue I see are possible error-messages of nc.
I'll apply the commit in a moment. Afterwords this can be marked as blocked by #2353 .

[christf]

this works as is as verified by test in Magdeburg.

One thing we should keep in mind is: babeld dump is CPU bound on large meshes and can have adverse effects on the performance of route distribution. After jech/babeld#57 is merged we should rework every component using dump on the babeld socked to reduce load.

[AiyionPrime]

@christf Thank you very much.