Add a utility to list missing shipits.

This is a tool to assist committers in finding review requests that
require their attention (i.e. when the request is addressed to you
explicitly and missing a public shipit from you). It uses the
ReviewBoard API via "./rbt api-get" (thus avoiding the need to wire in
RB creds).

Testing Done:
~aurora git aurora/. kts/missing-shipits
% ./build-support/list-missing-shipits kevints
% ./build-support/list-missing-shipits kevints | wc -l
       7

Reviewed at https://reviews.apache.org/r/17823/