Support wrapping process instead of reading from piped stdin

[renbou]

Loggo pigeon client should support running in a form like loggo pigeon -- launch app with params, which would launch the app as a child of the logging process, which will allow loggo to properly control graceful shutdown (pass it onto the child, then send all the buffered messages, etc) and to run without a tini init process, like it needs to right now.