factor: improve fd buffering

factor: improve fd buffering

* src/factor.c (struct lbuf_, lbuf, lbuf_alloc): Remove.
All uses removed.
(FACTOR_PIPE_BUF): Now a constant instead of a macro.
Increase to PIPE_BUF if available.
(lbuf_buf, lbuffered): New static vars, replacing lbuf.
All uses changed.
(lbuf_flush): Avoid unlikely recursion on write failure.
(lbuf_putc): Now simply adds a byte to the buffer.
(lbuf_putnl): Do the work of the old lbuf_putc ('\n').
Use changed.  Use memrchr to find the newline.
(lbuf_putint): Widths are now int, not size_t.