rename main module to zio.ko

Now that the default trigger and default buffer are includes,
the module shoud be called zio.ko. zio-core.c will be the non-sysfs
stuff currently in zio-sys.c

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>