[Groff] added curves in grn

[Daniel Senderowicz]

Hi guys,

When I was debugging xgremlin a few days ago, I noticed a somehow
obscure feature. You can draw in addition to 'pic' type of curves,
b-splines and bezier curves by pressing the shift key while you
press the middle button of the mouse. It is documented in xgremlin
when you get help (pressing the right button on the respective
icon). Looking at the ancient versions of grn I saw that they could
parse these too. Therefore I modified the groff-grn code to include
these options.  Werner, let me know when you want me to send the
updated code for grn.

Dan