mux_plan9.go: get it to build

[rminnich]

I don't know what was intended with Set and Clr for Plan 9. I don't understand what to do to finish up Cursor2 yet.

For the moment, just trying to figure out if making this work on Plan 9 is of interest.

Corrections to this are most welcome of course.

[mischief]

msg_plan9.go which defined the cursor structure with Set and Clr got deleted in da861c1. wrt the 'intention', graphics(2):

A Cursor is defined:

               typedef struct
               Cursor {
                     Point offset;
                     uchar clr[2*16];
                     uchar set[2*16];
               } Cursor;

          The arrays are arranged in rows, two bytes per row, left to
          right in big-endian order to give 16 rows of 16 bits each.
          A cursor is displayed on the screen by adding offset to the
          current mouse position, using clr as a mask to draw white at
          the pixels where clr is one, and then drawing black at the
          pixels where set is one.  Setcursor and moveto (see
          [mouse(2)](https://man.9front.org/2/mouse)) and esetcursor and emoveto (see [event(2)](https://man.9front.org/2/event)) change
          the cursor image and its location on the screen.

[rminnich]

do you have a CI that tests building, and if so, can it/does it test for Plan 9? Just trying to get some idea if you want that to work or not.