pppox: use standard module auto-loading feature

* Register PF_PPPOX with pppox module rather than with pppoe,
    so that pppoe doesn't get loaded for any PF_PPPOX socket.

  * Register PX_PROTO_* with standard MODULE_ALIAS_NET_PF_PROTO()
    instead of using pppox's own naming scheme.

  * While there, add auto-loading feature for pptp.

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