Fix bpf ffi and add OpenBSD bpf cfg

[pk-j-xyz]

This fixes the as_ptr call which currently is causing a dealloc before the FFI call to libc open. This resulted in a permanent "file not found" when opening any device.

OpenBSD 6.8 amd64. Rust 1.46.0. smoltcp 15f3bc5.

core::ptr::slice_from_raw_parts_mut::h6b6d2aa746578da4 (data=0xc5035c4c420 "/dev/bpf0\000", len=10)
alloc::alloc::dealloc::h4e59c1527d840d21 (ptr=0xc5035c4c420 "/dev/bpf0\000", layout=...)
alloc::alloc::dealloc::h4e59c1527d840d21 (ptr=0xc5035c4c420 '\337' <repeats 32 times>, "\000",
smoltcp::phy::sys::bpf::open_device::h9e20766127888bdc ()
_libc_open_cancel (path=0xc5035c4c420 '\337' <repeats 32 times>, flags=2)

This also adds OpenBSD support by adjusting the existing cfg options in bpf.rs.

Thanks for your work! smoltcp is awesome.