VSOCK: Move vmci_sockets.h to uapi (and rename)

This is user-visible, so let's move it to the right place.  Also, now that
we're trying to be transport-independent, call it something more appropriate:
vm_sockets.h, with all definitions changed to match.  Fix up the sources
accordingly.

Acked-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Andy King <acking@vmware.com>