Actually it's SIGINT that gracefully kills, nm.

Actually going to implement this, since it's the standard way to gracefully kill subprocesses (e.g., python's proc.terminate()).

The following revision refers to this bug:
  https://chromium.googlesource.com/external/github.com/luci/luci-go.git/+/cc57c52dba1c927bff470f8f3550beff41f33e46

commit cc57c52dba1c927bff470f8f3550beff41f33e46
Author: dnj <dnj@chromium.org>
Date: Sat Jul 02 16:05:23 2016

LogDog Butler: Catch SIGTERM on Linux.

TBR=stip@chromium.org
BUG= chromium:624964 
TEST=local
  - Ran a Butler instance, killed via "kill $PID", observed that it
    flushed.

Review-Url: https://codereview.chromium.org/2114253002

[modify] https://crrev.com/cc57c52dba1c927bff470f8f3550beff41f33e46/client/cmd/logdog_butler/main.go
[rename] https://crrev.com/cc57c52dba1c927bff470f8f3550beff41f33e46/client/cmd/logdog_butler/main_posix.go
[rename] https://crrev.com/cc57c52dba1c927bff470f8f3550beff41f33e46/client/cmd/logdog_butler/main_windows.go