Expose kernel-only errno values if _WANT_KERNEL_ERRNO is defined.

The kernel uses a few negative errno values for internal conditions
such as requesting a system call restart. Normally these errno values
are not exposed to userland. However, kdump needs access to these
values as some of then can be present in a ktrace system call return
record. Previously kdump was defining _KERNEL to gain access to ehse
values, but was then having to manually declare 'errno' (and doing it
incorrectly). Now, kdump uses _WANT_KERNEL_ERRNO instead of _KERNEL
and uses the system-provided declaration of errno.