ppp: define reusable device creation functions

Move PPP device initialisation and registration out of
ppp_create_interface().
This prepares code for device registration with rtnetlink.

While there, simplify the prototype of ppp_create_interface():

  * Since ppp_dev_configure() takes care of setting file->private_data,
    there's no need to return a ppp structure to ppp_unattached_ioctl()
    anymore.

  * The unit parameter is made read/write so that ppp_create_interface()
    can tell which unit number has been assigned.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>