Support custom wintun.dll paths

[blechschmidt]

Instead of the previous approach using an environment variable, this approach leaves it to the caller to specify a custom path to the wintun.dll.

This approach has the following advantages:

1. In contrast to the origin of information in environment variables, the security context of the origin data is clear.
2. There are no security-related changes for existing users of the library.

[ssrlive]

Good idea.