[Groff] pdfmark: spurious dropped links

[Larry Kollar]

In my continued attempts to get PDF links working properly, I'm now running 
into one of the most frustrating aspects of any troubleshooting job: the 
intermittent error. Cross-reference links that work in one place don't work in 
the other, and I haven't yet found a pattern. Looking at the reference file, 
the non-working links correspond to "pdfhref Z" pairs like this:

.pdfhref Z 252 475260 541000
.pdfhref Z 252 475260 541000

In other words, they begin and end at the same point. The broken links are 
consistent between runs, at least. Any thoughts?

-- Larry