If CLEARERRORKEY is set to BKSP, then msgs issued in one file appear
in all the other files in the ring! Although this can be a convenient feechur
on occasion, it is VERY disconcerting in split screen mode and prevents
the macro writer from using a msg as a semi-permanent prompt.
Besides which, the behavior with split vertical screens is inconsistent: not *every* msg is copied to *both* screens when you use NEXTWINDOW to go back and forth.
I think msgs should only appear in the window of the file that they were issued against, unless some MSGLINE option is provided, or a MSGALL command is added.