vpython: Add primary execution package.

vpython/system_posix.go

Index: vpython/system_posix.go
diff --git a/vpython/system_posix.go b/vpython/system_posix.go
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index 0000000000000000000000000000000000000000..c52f80e8a9069469bab39561d1654bb83d750d10
--- /dev/null
+++ b/vpython/system_posix.go
@@ -0,0 +1,22 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// +build darwin linux freebsd netbsd openbsd android
+
+package vpython
+
+import (
+	"os"
+	"syscall"
+)
+
+// forwardedSignals is the list of signals that should be forwarded to the
+// Python subproess.
+var forwardedSignals = []os.Signal{
+	syscall.SIGHUP,
+	syscall.SIGINT,
+	syscall.SIGQUIT,
+	syscall.SIGUSR1,
+	syscall.SIGUSR2,
+}