selftests: nettest: Add --{force,no}-bind-key-ifindex

These options allow explicit control over the TCP_MD5SIG_FLAG_IFINDEX
flag instead of always setting it based on binding to an interface.

Do this by converting to getopt_long because nettest has too many
single-character flags already and getopt_long is widely used in
selftests.

Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>