Handle running without administrator privileges.

If the service is configured to run as an unprivileged user NSSM will
not be able to perform certain tasks which are not critical and will
therefore log some errors which can be ignored.

It will not be able to register itself as an event log source.
It will not be able to create a default AppExit key.
It will not be able to configure service failure handling.

Try to perform these tasks where first installing the service (which
we are guaranteed to do with administrator rights) as well as at
runtime, and don't log the errors if the runtime actions can't be
performed.