core: add ExecMainHandoverTimestamp property recording time-of-execve

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.