systemd-user: Enable "systemctl --user reload {dirmngr,gpg-agent}"

* doc/examples/systemd-user/*.service: Add ExecReload directives to
  indicate the canonical way to reload the services.

GnuPG recommends reloading the agent and dirmngr with "gpgconf
--reload". if anyone is running them as systemd user services, they
might ask them to reload in the systemd way, so teach systemd the
right thing to do.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>