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

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

xen: switch to use headers in contrib

These headers originate with the Xen project and shouldn't be mixed with
the main portion of the FreeBSD kernel. Notably they shouldn't be the
target of clean-up commits.

Switch to use the headers in sys/contrib/xen.

Reviewed by: royger

xen: rewrite Xen error translation

Using the public Xen error code header.

Submitted by: Akshay Jaggi <akshay1994.leo@gmail.com>
Reviewed by: royger

xen: add a Xen to BSD error translation

Since Xen and FreeBSD error codes are completely different add a
translation layer in order to convert Xen error codes into native
FreeBSD error codes. This will be used by the privcmd device, which
needs to return the hypercall errors into user-space.

Sponsored by: Citrix Systems R&D

xen/error.h:
- Import Xen error codes.
- Create a table to map Xen error codes into FreeBSD native error
codes.
- Create an inline function that performs the translation.