sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Update VirtIO definitions from Linux virtio_config.h and virtio_ring.h headers

Move the VIRTIO_RING_F_* defines out of virtqueue.h into virtio_config.h

These defines are applicable to userland too, but virtqueue.h contains
the kernel virtqueue interface, and is therefore not usable in userland.

Note that Linux places these defines in virtio_ring.h, but I don't want
the drivers including this header file to keep the VirtIO ring opaque to
everything but the virtqueue.

MFC after: 1 week

Split the virtio.h header file into multiple files

Reorganize the previous contexts of the file as it is in Linux. The
eventual goal is to install the header files and share them between
the kernel and bhyve.

MFC after: 1 week