init: Add more diagnostics for b/223076262.

This adds three more diagnostics to stuck exec services:

1. /proc/pid/fds is dumped
2. /proc/pid/status is dumped
3. HandleSignalFd is called to see if a SIGCHLD got stuck somewhere

Bug: 223076262
Test: while (1) in linkerconfig
Ignore-AOSP-First: diagnostics
Change-Id: Ida601d86e18be9d49b143fb88b418cbc171ecac6
diff --git a/init/service.cpp b/init/service.cpp
index 2ebf87e..48688f5 100644
--- a/init/service.cpp
+++ b/init/service.cpp
@@ -127,6 +127,7 @@
 
 unsigned long Service::next_start_order_ = 1;
 bool Service::is_exec_service_running_ = false;
+pid_t Service::exec_service_pid_ = -1;
 std::chrono::time_point<std::chrono::steady_clock> Service::exec_service_started_;
 
 Service::Service(const std::string& name, Subcontext* subcontext_for_restart_commands,
@@ -389,6 +390,7 @@
 
     flags_ |= SVC_EXEC;
     is_exec_service_running_ = true;
+    exec_service_pid_ = pid_;
     exec_service_started_ = std::chrono::steady_clock::now();
 
     LOG(INFO) << "SVC_EXEC service '" << name_ << "' pid " << pid_ << " (uid " << proc_attr_.uid