init start time tracking.

With this change, init sets a property "init.start" to show the
CLOCK_BOOTTIME time at which init itself started, and for each service
an "init.svc.<name>.start" property to show the CLOCK_BOOTTIME time at
which that service was most recently started.

These times can be used by tools like bootstat to track boot time.

As part of this change, move init over to std::chrono. Also, rather than
make the command-line argument handling more complex, I've switched to
using an environment variable for communication between first- and
second-stage init, and added another environment variable to pass the
start time of the first stage through to the second stage.

Bug: http://b/32780225
Test: manual
Change-Id: Ia65a623e1866ea688b9a5433d6507926ce301dfe
diff --git a/init/util.h b/init/util.h
index 12ab173..dccec04 100644
--- a/init/util.h
+++ b/init/util.h
@@ -20,11 +20,14 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
+#include <chrono>
 #include <string>
 #include <functional>
 
 #define COLDBOOT_DONE "/dev/.coldboot_done"
 
+using namespace std::chrono_literals;
+
 int create_socket(const char *name, int type, mode_t perm,
                   uid_t uid, gid_t gid, const char *socketcon);
 int create_file(const char *path, int mode, mode_t perm,
@@ -33,27 +36,35 @@
 bool read_file(const char* path, std::string* content);
 int write_file(const char* path, const char* content);
 
-time_t gettime();
-uint64_t gettime_ns();
+// A std::chrono clock based on CLOCK_BOOTTIME.
+class boot_clock {
+ public:
+  typedef std::chrono::nanoseconds duration;
+  typedef std::chrono::time_point<boot_clock, duration> time_point;
+  static constexpr bool is_steady = true;
+
+  static time_point now();
+};
 
 class Timer {
  public:
-  Timer() : t0(gettime_ns()) {
+  Timer() : start_(boot_clock::now()) {
   }
 
   double duration() {
-    return static_cast<double>(gettime_ns() - t0) / 1000000000.0;
+    typedef std::chrono::duration<double> double_duration;
+    return std::chrono::duration_cast<double_duration>(boot_clock::now() - start_).count();
   }
 
  private:
-  uint64_t t0;
+  boot_clock::time_point start_;
 };
 
 unsigned int decode_uid(const char *s);
 
 int mkdir_recursive(const char *pathname, mode_t mode);
 void sanitize(char *p);
-int wait_for_file(const char *filename, int timeout);
+int wait_for_file(const char *filename, std::chrono::nanoseconds timeout);
 void import_kernel_cmdline(bool in_qemu,
                            const std::function<void(const std::string&, const std::string&, bool)>&);
 int make_dir(const char *path, mode_t mode);