Rename ROWS to LINES (it's what VE uses). Also, don't use COLS and
LINES as immediates in the Grid subsystem: we expect those words to
be available at runtime.
This is more than cosmetic, it's also highly usable. The presence
or absence of the "ok" message allows us to know whether the command
aborted. Previously, the "> " prompt appeared when the system expected
a prompt in the INTERPRET context, whether the previous command aborted
or not.
Also, this allows us to get rid of that ugly FLAGS global variable.
