wnbd-client: use utf-8

[petrutlucian94]

wnbd-client doesn't currently accept unicode CLI arguments.

We'll use utf-8 instead of utf-16 for the following reasons:

• better interoperability
• the wnbd API is mostly using char instead of wchar
• we already require Windows >= 10.0.17134.0, so we're free to use the utf-8 locale