selftests/bpf: Parametrize AF_UNIX redir functions to accept send() flags

Extend pairs_redir_to_connected() and unix_inet_redir_to_connected() with a
send_flags parameter. Replace write() with send() allowing packets to be
sent as MSG_OOB.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20240713200218.2140950-4-mhal@rbox.co