factor: ensure atomic output through pipes

factor: ensure atomic output through pipes

The new tests/misc/factor-parallel.sh test was
seen to fail on FreeBSD (derived) systems, which was
due to split(1) --filter reading partial lines
through pipes, as factor(1) was writing a little
over PIPE_BUF each time.

* src/factor.c (lbuf): A new structure to internally buffer lines.
(lbuf_alloc): A new function to allocate enough at program start.
(lbuf_putint): A new function to buffer a uintmax_t.
(lbuf_flush): A new function to write directly to standard output.
(lbuf_putc): A new function to buffer a character and if enough
lines are buffered, then output complete lines <= PIPE_BUF,
and continue to buffer the rest.
(main): Call the internal buffer allocator, and register
the final flush from the internal buffer at program exit.