Use nssm_imagepath().

We were calling GetModuleFileName() in a number of places.  Use
nssm_imagepath() instead.

Ensure that a quoted path is used for the service image path when
creating the service, thus avoiding a theoretical security vulnerability
whereby the service manager could be tricked into launching the wrong
program.

When setting the path to NSSM in the environment for event hooks we
still use the unquoted path, as environment variables are typically
unquoted.

Thanks Gerald Haider.