core: add ExecMainHandoverTimestamp property recording time-of-execve

Enable the exec_fd logic for Type=notify* services too, and change it
to send a timestamp instead of a '1' byte. Record the timestamp in a
new ExecMainHandoverTimestamp property so that users can track accurately
when control is handed over from systemd to the service payload, so
that latency and startup performance can be trivially and accurately
tracked and attributed.

src/core/execute.h

@@ -91,6 +91,7 @@ typedef enum ExecKeyringMode {
 struct ExecStatus {
         dual_timestamp start_timestamp;
         dual_timestamp exit_timestamp;
+        dual_timestamp handover_timestamp;
         pid_t pid;
         int code;     /* as in siginfo_t::si_code */
         int status;   /* as in siginfo_t::si_status */
@@ -443,7 +444,9 @@ struct ExecParameters {
         int stdout_fd;
         int stderr_fd;
 
-        /* An fd that is closed by the execve(), and thus will result in EOF when the execve() is done */
+        /* An fd that is closed by the execve(), and thus will result in EOF when the execve() is done. It
+         * will also be used to send a timestamp taken as the very last operation before execve, for
+         * tracking purposes. */
         int exec_fd;
 
         char *notify_socket;