wg: ipc: add allowed-ip flags support for FreeBSD

For $reasons, we can't rely on flags in wireguard-tools for the kernel
side of WireGuard. Provide a mapping function that uses flags from the
kernel that we're building against and fail the operation if we made it
to the end without turning some wg(8) flag off.

Signed-off-by: Kyle Evans <kevans@FreeBSD.org>

Reviewed by: ivy (previous version), Jason A. Donenfeld, jhb
Differential Revision: https://reviews.freebsd.org/D50450