Add support for specify `libbpf_flags`?

[ihciah]

By default setting libbpf_flags to 0 will make libbpf load its ebpf prog. But for many cases we don't want all the packets to userspace. For example, maybe we only want port 80 packets. In this situation we can do the filtering in ebpf side.

To load custom ebpf prog we need supply XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD to libbpf_flags.

Will it be acceptable to add a libbpf_flags field for SocketOptions? Since it's a pub struct with pub fields, it's a breaking change. If its okay, I will submit a PR.