Use carriage returns for data insertion logs in pgbench on terminal

Use carriage returns for data insertion logs in pgbench on terminal

This is similar to what pg_basebackup and pg_rewind do when reporting
cumulative data, and that's more user-friendly.  Carriage returns are
now used when stderr points to a terminal, and newlines are used in
other cases, like a redirection to a log file.

Author: Amit Langote
Reviewed-by: Fabien Coelho
Discussion: https://postgr.es/m/CA+HiwqFNwEjPeVaQsp2L7DyCPv1Eg1guwhrVhzMYqUJUk8ULKg@mail.gmail.com