rust-chg: add config validation and process returned instructions This is the reimplementation of runinstructions() and main() in chg.c. In Rust version, we only pass in early arguments to the server as the locator doesn't know the full arguments. This should be fine since these arguments are just passed in to _earlyparseopts() and _parseconfig(), which means the server doesn't need full arguments. Another difference is the handling of the "exit <code>" instruction. In Rust version, we can simply reuse the connection instead of "exit(code)" as the command error isn't displayed yet. That's because the client-side stdio is not attached until the connection is validated. This behavior is cleaner than C, but it also means that the early server exception wouldn't be propagated to client because stderr isn't attached. So we might have to reconsider when to attach/detach the server stdio. Differential Revision: https://phab.mercurial-scm.org/D8381