\D'R' support

Hi all,

I've recently discovered that groff's output drivers grodvi, grolj4, and
grolbp support an additional drawing command \D'R'. I notice from the
respective manpages that the reason for its addition seems to be lack of
support for other drawing commands by the respective devices.

With that said, I've been wondering why it's device-specific and not
also supported by grops and gropdf despite being easy to implement in
PostScript and PDF?

It seems like a useful command to replace the old \l (that uses \[ru])
since \D'l' is specified as having round line endings by default.

Cheers,
onf