BUG/MEDIUM: log: don't mark log FDs as non-blocking on terminals

BUG/MEDIUM: log: don't mark log FDs as non-blocking on terminals

With the new ability to log to a terminal, it's convenient to be able
to use "log stdout" in a config file, except that it now results in
setting the terminal to non-blocking mode, breaking every utility
relying on stdin afterwards. Since the only reason for logging to a
terminal is to debug, do not set the FD to non-blocking mode when it's
a terminal.

This fix must be backported to 1.9.