pg_upgrade log files

Right now, the pg_upgrade output files contain the output of the
commands it runs. However, it does not contain the actual commands
being run ... so if you want to figure out what is happening, you have
to trace the pg_upgrade source while reading the log file.

This is, to say the least, suboptimal.

I propose this patch which echoes the commands to the respective log
files. I would backpatch this to 9.2.

As a note, I find the exec_prog() function rather bizarre. It would be
better if the caller did not have to manually specify a redirection, for
instance.

--
Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>