orphan .Xr in manual pages

[Yuri Pankov]

Poul-Henning Kamp wrote:
> --------
> In message <30799.1586023332 at critter.freebsd.dk>, "Poul-Henning Kamp" writes:
> 
>> The script was attached to the email, and I'd love for somebody to adopt it,
>> I just wanted to see how bad the problem was...
> 
> I guess that got stripped by mailman, here it is:
> 
> 	http://phk.freebsd.dk/misc/man_xref.sh

Here's a bit shorter version using the mandoc's lint:

for m in `find /usr/share/man -type f`; do
     mandoc -Tlint $m | grep 'referenced manual not found'
done

Running this on installed system produces nearly the same results as in 
the original mail.  We could enhance mandoc to make the missing .Xr 
target get its own warning level, to further enhance this.