RUBOUT function

[GSB at MIT-MC (Glenn S. Burke)]

The rubout function, if the horizontal cursor position is less than
the number of positions which are to be killed, does a (CURSORPOS 'U),
apparently neglecting the fact that doing a (CURSORPOS 'B) will do a
(CURSORPOS 'U) when it goes past the left margin.
It should either do a (CURSORPOS 'X) the proper number of times,
or just (CURSORPOS 'B) the proper number of times, followed by 
(CURSORPOS 'L).  (The former is 'correct', but the latter will bypass
an its bug...)
The current method ends up in the proper (usually) position, but on the
wrong line.  Also, this method of rubbing chars out is ineffective anyway
if a reprint of the buffer inserts terpri's when it tyo's the characters.