fix: use CString for interface name

[dzania]

When building for freebsd code won't compile due to

    Checking defguard_wireguard_rs v0.3.0 (/home/dzania/teonite/linux_route/wireguard-rs)
error[E0308]: mismatched types
    --> src/bsd/ifconfig.rs:52:22
     |
52   |             kld_load(mod_name.as_ptr());
     |             -------- ^^^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`
     |             |
     |             arguments to this function are incorrect
     |
     = note: expected raw pointer `*const i8`
                found raw pointer `*const u8`
note: function defined here
    --> /home/dzania/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libc-0.2.150/src/unix/bsd/freebsdlike/freebsd/mod.rs:5539:12
     |
5539 |     pub fn kld_load(name: *const ::c_char) -> ::c_int;

Fix for this is using CString from std::ffi