selftests: Add separate options for server device bindings

Add new options to nettest to specify device binding and expected
device binding for server mode, and update fcnal-test script. This
is needed to allow a single instance of nettest running both server
and client modes to use different device bindings.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>