Added NETCONFIG override by environment variable.

[yonic2]

The NETCONFIG location can be overidden by setting the "NETCONFIG_OVERRIDE" environment variable. This can facilitate unit testing.

[dang]

Why do you want to override the netconfig file location?

[yonic2]

It's needed for unit testing. In some testing environments the file doesn't contain what libntirpc expects.

[dang]

Would a CMake compile-time option be suitable?