Prepare the terminal driver for filling multiple columns in parallel,

Prepare the terminal driver for filling multiple columns in parallel,
second step: make the per-column byte pointer persistent across
term_flushln() calls, such that a subsequent call can continue at
the point where the previous call left.  If more than one column
is in use, return from term_flushln() when the column is full,
rather than breaking the output line.

No functional change, because nothing sets up multiple columns yet.