gaby: initial main loop

Copied from https://github.com/rsc/gaby/commit/2a39bf8.

Change-Id: I12aa6e743fc424c6cebfd03df0a03e30bc105c3c
Reviewed-on: https://go-review.googlesource.com/c/oscar/+/597152
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>