Prevent a counter buffer overrun when numbering lines and when

Prevent a counter buffer overrun when numbering lines and when
processing 100 billion lines (or more) of input.

(LINE_COUNTER_BUF_LEN): Define to allow numbering as
many as 10^18 - 1 lines (the old limit was 10^11 - 1, and could
be exceeded without too much trouble).	Use this symbol rather
than hard-coding the constant everywhere.  Rather than overruning
for input with more lines, mark the line number by putting a
`>' in the leftmost slot.
(next_line_num): Fixed (now academic) possible line buffer overrun.
Patch by Jan Nieuwenhuizen.