userspace reboot: stop post-data services and wait for them to be killed

* Refactored code around stopping services a little bit to reuse it
  between full reboot and userspace reboot.
* Add a scope_guard to fallback to full reboot in case userspace reboot
  fails.
* In case of userspace reboot init will also wait for services to be
  terminated/killed and log the ones that didn't react to
  SIGTERM/SIGKILL in time.
* If some of the services didn't react to SIGKILL, fail userspace reboot.

Test: adb reboot userspace
Bug: 135984674
Change-Id: I820c7bc406169333b0f929f0eea028d8384eb2ac
diff --git a/init/sigchld_handler.h b/init/sigchld_handler.h
index 30063f2..fac1020 100644
--- a/init/sigchld_handler.h
+++ b/init/sigchld_handler.h
@@ -17,11 +17,16 @@
 #ifndef _INIT_SIGCHLD_HANDLER_H_
 #define _INIT_SIGCHLD_HANDLER_H_
 
+#include <chrono>
+#include <vector>
+
 namespace android {
 namespace init {
 
 void ReapAnyOutstandingChildren();
 
+void WaitToBeReaped(const std::vector<pid_t>& pids, std::chrono::milliseconds timeout);
+
 }  // namespace init
 }  // namespace android