dpdk: New module with some code from netdev-dpdk.

dpdk: New module with some code from netdev-dpdk.

There's a lot of code in netdev-dpdk which is not at all related to the
netdev interface, mostly the library initialization code.

This commit moves it to a new 'dpdk' module, to simplify 'netdev-dpdk'.

Also a new module 'dpdk-stub' is introduced to implement some functions
when DPDK is not available.  This replaces the old 'netdev-nodpdk'
module.

Some redundant includes are removed or reorganized as a consequence.

No functional change.

CC: Aaron Conole <aconole@redhat.com>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Tested-by: Aaron Conole <aconole@redhat.com>