cli: add a flag to allow listening on all interfaces

[tamird]

This permits users to use the --host and --port flags to control the
node's advertised external address while listening on all interfaces.

Fixes #1008.

cc @mberhault

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This change is

[bdarnell]

See also #5816, which is a request for a way to bind to multiple specific interfaces (but not all of them). I think it may be better to go ahead and implement a list of secondary addresses to bind instead a special case to bind to everything.

[tamird]

FYI it's not so simple (golang/go#9334), so the multiple --host flags suggested in #5816 won't work.

[petermattis]

@tamird Can't you create a new listener for each address you want to listen on? You could probably wrap that up in a MultiListen method that does the multiplexing internally.

[mberhault]

what does this give us over the use of --host=0.0.0.0?

[petermattis]

@mberhault You can specify exactly which interfaces you listen on.

[mberhault]

sorry, I meant what does this current PR give us that 0.0.0.0 doesn't? we're still only allowing one interface or all.

[tamird]

right, but --host currently controls the advertised address as well as the listening address

[mberhault]

sure, but that has nothing at all to do with what we listen on. I would prefer to see --host control which interface(s) we bind to, and a separate flag overriding our advertised address.

[tamird]

Updated, PTAL.