net/tap: Replace tap-haiku.c and tap-aix.c by a generic tap-stub.c

The files tap-haiku.c and tap-aix.c are identical (except one line
of error message). We should avoid such code duplication, so replace
these by a generic tap-stub.c file instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>