Ability to limit inbound unannounced channels

[npslaney]

Similar to #675 , being able to have some policy around the types of channels others open to you when using ldk-node as a public routing node is useful. Disallowing inbound unannounced channels is another setting routing nodes use.

[TheBlueMatt]

Somewhat-dup of #502. Ultimately probably needs some kind of generic "dont accept channels automatically, let me do it manually" thing.