Do not seemingly #include Linux-specific headers on other platforms.

Do not seemingly #include Linux-specific headers on other platforms.

Until now, the OVS source tree has had a whole maze of header files that
make "#include <linux/openvswitch.h>" work OK regardless of platform, but
this confuses everyone new to the tree, at first glance, and is difficult
to understand at second glance too.

This commit renames include/linux/openvswitch.h to
datapath/linux/compat/include/linux/openvswitch.h without other change,
then modifies the userspace build to generate a header that makes sense in
portable Open vSwitch userspace from that header.

It then removes all the remaining include/linux/* files since they are now
unused.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>