If iface_bind() fails, that's a hard error.

If iface_bind() fails, that's a hard error.

If you can open a PF_PACKET socket, you don't need to fall back on
PF_INET/SOCK_PACKET sockets, and we've already opened that socket by the
time we call iface_bind().

Have iface_bind() return 0 on success and a PCAP_ERROR_ value on
failure.  If the bind() call fails with ENODEV, return
PCAP_ERROR_NO_SUCH_DEVICE, otherwise return PCAP_ERROR.

If getsockopt(SO_ERROR) fails, that's an error; if it succeeds, and
supplies a pending error code, that's an error, too.