zmirror: Allow duplicate zmirror processes to die gracefully.

Fixes #602.

I replaced the SIGKILL with a SIGINT, and then catch SIGINT with a
handler.  This handler calls cancelSubs if necessary, and can later be
edited to perform other clean-up operations, too. I thought about, in
this same commit, changing the SIGTERM in
maybe_restart_mirroring_script to a SIGINT, but after tracing out the
code paths, I realized that isn't necessary. (The SIGTERM is
necessarily performed on a process that has not subscribed to any
zephyr classes, so cancelSubs is unnecessary. If we do think that we
may want to add additional clean-up operations in the future, though,
then it might be worth investigating changing this SIGTERM.)

(imported from commit 692b295be6cb40b0e4ec2ca0bc58c58056ed9bd9)

bots/zephyr_mirror.py

@@ -27,9 +27,16 @@ import time
 import optparse
 import os
 import traceback
+import signal
 
 from zephyr_mirror_backend import parse_args
 
+def die(signal, frame):
+    # We actually want to exit, so run os._exit (so as not to be caught and restarted)
+    os._exit(1)
+
+signal.signal(signal.SIGINT, die)
+
 (options, args) = parse_args()
 
 args = [os.path.join(options.root_path, "user_root", "zephyr_mirror_backend.py")]