selftests: fib_rule_tests: Test UDP and TCP connections with DSCP rules.

Add the fib_rule6_send and fib_rule4_send tests to verify that DSCP
values are properly taken into account when UDP or TCP sockets try to
connect().

Tests are done with nettest, which needs a new option to specify
the DS Field value of the socket being tested. This new option is
named '-Q', in reference to the similar option used by ping.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>