factor: output immediately if stdout is a tty but stdin is not

factor: output immediately if stdout is a tty but stdin is not

* src/factor.c (lbuf_putc): Use line buffered mode if the standard
output is a terminal in the same way as the stdio library.
User programs might use pty only for the standard out
like the example of Ruby's PTY module:
https://docs.ruby-lang.org/en/2.6.0/PTY.html#module-PTY-label-Example
* NEWS: Mention the fix.
Fixes https://bugs.gnu.orv/35046